As I've said countless times before, think about which pokemon would be your opponent interested in taking out to plow a hole and open a sweep. Then after Gengar has Megavolved and switched out the first time, predict when MGar will come in and double switch to keep it at bay while you neuter your real check. With any kind of hazards or sticky web it will finally take care of either of them. Not mentioning that every time the Scarfed ghost counter switches in to take out MGar, it takes damage.
Actually, in some situations I can understand what you explained being good for the metagame. Why? Because im one of those guys that think Dugtrio / Maggy in ADV is great and a very important part of the metagame that adds a ton of skill. Here lets do a little backstory:
Dugtrio / Mag are Pokemon that offer a shitton of utility in ADV. Specifically on Dugtrio, you never, ever, bring Dugtrio in as a switch unless its a ~predicted~ double switch from you. Because hes frail as shit, duh, everything would kill it. This presence in ADV is good from my point of view because it causes people to think. You cant just leave your Skarmory in throwing spikes all the time because theres the chance of a Magneton coming in and fucking you up. You cant kill something with Raikou for shits and giggles because Dugtrio comes in and shits on you. You have to think. You have to think about if the move youre about to do is worth the risk of losing the mon entirely to Dugtrio or Mag.
This is literally what you just described up there. The difference are three:
1) In ADV this offensive presence is needed. Defensive teams are very damn strong in ADV due to the lack of nuts offensive options like the ones in BW / XY. Magneton and Dugtrio offer a way for offensive teams to deal with defensive teams if they require so. On the same vein, its good in order to keep the flow of an offensive game going.
This amount of offensive presence to the degree of Mega Gengar is NOT NEEDED.
2) The important one.
Look at what those 2 mons can trap. Between 2 mons, they can trap a total of around 4~ (Forry, Skarmory, Metagross ONLY IF LOW, and Steelix if theres HP Fire) and 7 with one repeat (Tyranitar, Raikou, Heracross, Blissey, Celebi ONLY IF WEAKENED, Jirachi, Metagross ONLY IF WEAKENED). Thats 10 mons. 10 mons that a combo of TWO can take out, and there needs to be previous damage on 4 of them. Compare this with Mega Gengar who can take on
OVER 70 POKEMON BY ITSELF.
3) Trace and revenge trapping exists in ADV. Porygon 2 can deal with non Substitute Magneton and any Dugtrio all of the time.
This doesnt work at all for Gengar, as he cant be trapped back and Pursuit users risk taking a Destiny Bond to the face (or OHKOd by Focus Blast in case of Ttar).
Lets go back to you, yes, what you said is something we theoretically should consider. Notice the choice of words. Theoretically should. But we wont. Its not happening. The situation you just described THE single most unhealthy metagame ive ever seen described. Lets exemplify it.
I send out Ferrothorn, opponent sends out Volcarona. Well shit, I have to switch out, I dont wanna lose Ferrothorn! I go to Blissey / Heatran / Terrakion. Three mons that can take on the Volcarona at +1. Now, this is a normal battle scenario in which the player is trading a free turn for Volcarona in order to be able to take it on the turn after, either netting himself a KO or a free turn for himself. This is standard battle procedure. Its what is supposed to happen.
This all changes because of Mega Gengar. Now im fucked. Any move I make here is a checkmate position for my opponent. Why? Because if I were to stay in in order to get something like, say, Scarf Landorus in, who cant switch in, but will take it on even at +1, id have to sacrifice Ferrothorn. Thats not an option for me, so my only other move would be bringing my Volcarona counter. Any of those 3 mons above. Now thats not an option either.
I bring in any of the 3 as Volcarona dances / uses anything. Now I have an option. I can take down Volcarona. Except if my opponent has some death fodder, he will sacrifice that, bring Mega Gengar in, and suddenly my Volcarona counter is gone. Now Volcarona instantly wins the moment it comes in because I have no way to stop it. The only move I can do in order to *STAND A CHANCE IN THIS GAME* would be to double switch when Volcarona is out, PREDICTING THE GENGAR SWITCH IN, in order to be able to take it down before it fucks me up. This sounds all nice and fun until you realize that
youll have to do this with 70 pokemon. So if my team has 3 of the mons that Gengar can trap and kill, I have to consider double switching every single time theyre out in order to not lose them and be potentially swept. And this all comes with its drawbacks of course, as if my opponent doesnt switch out I just lost a shitton of momentum or in the worst case a full mon. This becomes even worse when you UNDERSTAND that any good player
(WHICH THERE ARE LIKE NONE OF ON THE LADDER SRSLY STOP THINKING BECAUSE xxxHaxMaster420xxZ SUCKS AT GENGAR EVERYONE WILL) will never, ever, switch to Gengar giving you a free move on it. Never. Thats not gonna happen. Stick it out of your head. They will sac something and bring Gengar in, now youre truly fucked.
There is 0 counterplay to this. If you want to compare Mgar to someone, compare it with DEOXYS, not with Salamence or whatever sweeper you want. Deoxys in Gen 4 was a monster. You wouldnt switch it in unless you were 100% certain it would not get fucked in the process (for example, on something locked on a psychic move while the opponent has no good 1v1 mons for Deoxys so the chance of double switch is gone), YOU REVENGE KILLED WITH IT. Deoxys would come in after something in your team died, and instantly pose a huge threat to the opponent's team. FOR A DIFFERENT REASON THAN GENGAR TOO.
In gen 4, Deoxys had the ability to revenge kill almost anything in the game, and if you switched out, Deoxys would be able to 2HKO a large part of the metagame too. So you were left stranded wether you prefered losing the mon you had out, or potentially losing the mon you brought in.
MEGA GENGAR DOESNT EVEN GIVE YOU THAT CHOICE, FRIENDS. Mega Gengar will, like Deoxys, come in after a mon has died, AND KILL the mon thats out. Who gives a bleeding fuck if you have your super donger leet set of Mega Blastoise with Dark Pulse to take on Mega Gengar. YOU WONT GET TO BRING IT IN BEFORE GENGAR KILLS SOMETHING. That is fucking huge. Mega Gengar has no counter because you dont get the fucking chance to bring the counter in. The accepted definition of counter is something that can switch in with impunity to another mon and instantly threaten it.
SWITCH.IN. So can you please fucking tell me, how in the hell do you counter something that doesnt let you do the DEFINING PART OF A COUNTER. You dont. You cant.
I wasn't saying Gengar is similar to Keldeo, just the arguments but if it makes it any better here's a repost:
"Gengar has always the right moves and the right items and switch in situations at the right times while the opponent is always exposed and has nothing to switch to and the opponent has neither priority, status, abilities or pokemon that can stop it's sweep or check it/counter.
Except you ignored the part when I posted a specific set that I used for my list, with the exception of like 3 mons. So learn to read holy shit.
Also your Kangaskhan list is a joke. I purposely set you up on that one, because never will you be able to make a list like the one I made for Mega Gengar simply because theres nothing like Mega Gengar in terms of Shadow Tag + Power + Moveset. The list I made works for a simple reason, you dont get to switch out. With Kangaskhan, Garchomp, Heracross, Ampharos, Charizard, whatever the fuck you want, theres the chance of a switch so a list like that is pointless. Kangaskhan might beat those mons 1 on 1 but the opponent can simply bring in another one. This means you have to predict, risk vs reward, yada yada. With Mega Gengar this doesnt exist, Mega Gengar comes in, youre fucked, you lost a mon. Get this inside your skull already.
Unfortunately I have only played two styles in the current pokebank and OU metas, and either were A more defensively inclined team with a pursuit trapper for gengar, or offensive teams with focus on priority, not defense to check sweepers.
I can also say Mega Gengar is weak if I use a team full of shit that cant be trapped. But guess what, that means you cant use 70 mons in that list. Also did you know no priority move but Sucker Punches can OHKO him? So im glad your Azumarill weakened my Mega Gengar in exchange of its life, now my Talonflame or w/e can sweep with impunity. I chose Azumarill out of all the mons in your team to kill with Mega Gengar, and I set up a sweep accordingly. On the same vein if I was doing a say, DD Tyranitar sweep, id trap and Destiny Bond the Scizor. Or if I wanted a Dragon sweep, id trap your Mamoswine. I get to pick what im going to sweep you with and theres nothing you can do about it but never bringing the mon in question in, lest it risks being killed by Mega Gengar after I sac something.
Btw I made this for you people, cuz im nice
if u could all memorize this until u live to preach these sentences thatd be great