Well for mega pinsir to be suspected it would have to arguably sweep 3 types, and having used bug a bit I could at least suggest some types that mega pinsir could easily sweep.I strongly disagree that Grass is on the same level as Ice. Unlike Ice, Grass has access to at least one amazing mega, is not weak to Stealth Rock, and is not weak to some of the most common priority moves in the metagame unless you count Ice Shard, which is easily walled by Mega Venusaur and Ferrothorn anyway. Unfortunately Ice is in a situation in which you would need to ban a quarter of the metagame to make it good. Grass, on the other hand, really only has one mon that completely steamrolls it: Mega Pinsir (notice I said "completely". I realize Grass has a tough time against other types and mons, but at least it has relatively reliable checks to those other threats). Yes, Grass has a shaky check in the Rotom-Mow set that Azelea mentioned, but Ice has a shaky check for Scizor too in Specs Walrein. And no one is going to say that Ice has much of a chance against Scizor even with that Walrein.
Grass may not be a top type with Mega Pinsir gone, or even a mid-range type, but it would have the opportunity to be much more viable than it currently is. Ice would stay at the bottom even if Scizorite were banned.
I'm not outright saying that Pinsirite needs to be banned. I just think that out of all the mons on the table right now, it's at least in the top 3 for pokemon that should be suspected. I do want it to be suspected, but only because it deserves to be put on the chopping block and judged by the community. If the community decides it should not be banned, then I won't have any objections and I'll be satisfied that it was at least put to the test.
Now for me to consider a type as pretty much auto lose to one certain pokemon is when that type has only around 2 viable pokemon to stop said pokemon from obtaining a sweep.
First, as has already been mentioned, is fighting, which I feel has only two big answers to a +2 mega pinsir, cobalion and terrakion.
Cobalion pretty much only checks mega pinsir if its running focus sash or a lot of defense evs, since:
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 240+ Def Cobalion: 300-354 (77.7 - 91.7%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Mega Pinsir Close Combat vs. 252 HP / 240+ Def Cobalion: 362-426 (93.7 - 110.3%) -- 62.5% chance to OHKO
+2 252 Atk Mega Pinsir Earthquake vs. 252 HP / 240+ Def Cobalion: 302-356 (78.2 - 92.2%) -- guaranteed 2HKO after Leftovers recovery
252/240+ Cobalion was the most used physically defensive Cobalion that could take a hit from most mega pinsir, sitting at 4.8% of all Cobalion.
As you can see Mega Pinsir deals tons of damage even to defensive Cobalion, and that relies on it not having gotten prior damage done to it, which forces you to use something else as your lead unless:
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Terrakion: 211-250 (65.3 - 77.3%) -- guaranteed 2HKO
Terrakion can be run as a scarf to check Mega Pinsir, while not countering it scarf Terrakion can force out a Mega Pinsir, though this is assuming no prior damage has been done to Terrakion.
I feel like Mega Pinsir forces fighting users to use one of those two sets, as its easy to sack something and bring in and set up Mega Pinsir to set up since it resists fighting 4x, and 65/120 bulk it can survive most SE non STAB hits once, but one swords dance is all it takes to let Mega Pinsir sweep, and after +2 nothing is surviving return or quick attack unless you have a focus sash available, which can be difficult due to the lack of hazard removal on fighting monotypes, and the relative ease of setting up one stealth rocks with shuckle or ferrothorn, and combine this with some shuckle carrying a mental herb (42.4% of all shuckle on bug monotypes) and keeping a sash intact is easier said than done.
Another monotype that arguably has a hard time with Mega Pinsir could be water, since +2 return or close combat/earthquake ohkos most water pokemon.
While water pokemon don't have any counters that switch into Mega Pinsir more than once, the biggest threat to Mega Pinsir on water monotypes is probably Rotom-W, who can either run a fast scarf spread to Will o Wisp or Volt Switch and chip away mega pinsir and defensive spreads who can tank a hit.
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Rotom-W: 169-200 (55.5 - 65.7%) -- guaranteed 2HKO after Leftovers recovery
(This spread is 10.3% of Rotom-W)
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Rotom-W: 91-108 (37.7 - 44.8%) -- guaranteed 3HKO after Leftovers recovery
(The choice scarf spread, 16.2% of all Rotom-W)
The main saving grace of water monotypes is that scald and ice moves are everywhere and it has multiple ways of chipping Mega Pinsir, but once Mega Pinsir gets rolling it gets pretty hard to stop, especially with chip damage from the bug team and stealth rock support.
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Slowbro: 334-394 (84.7 - 100%) -- 6.3% chance to OHKO
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Slowbro: 334-394 (84.7 - 100%) -- 81.3% chance to OHKO after Stealth Rock
252+ Atk Mega Swampert Ice Punch vs. 0 HP / 0 Def Mega Pinsir: 170-202 (62.7 - 74.5%) -- guaranteed 2HKO
+2 252 Atk Aerilate Mega Pinsir Return vs. 100 HP / 0 Def Mega Swampert: 456-537 (124.5 - 146.7%) -- guaranteed OHKO
+2 252 Atk Mega Pinsir Close Combat vs. 252 HP / 252+ Def Lanturn: 298-351 (65.6 - 77.3%) -- guaranteed 2HKO after Leftovers recovery
(Assuming Mega Pinsir chose CC over earthquake)
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 4 Def Mega Sharpedo: 258-304 (91.8 - 108.1%) -- 50% chance to OHKO
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 4 Def Sharpedo: 390-460 (138.7 - 163.7%) -- guaranteed OHKO
252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Unaware Quagsire: 198-234 (50.2 - 59.3%) -- 79.3% chance to 2HKO after Leftovers recovery
+2 252 Atk Aerilate Mega Pinsir Return vs. 120 HP / 248+ Def Alomomola: 412-486 (82.2 - 97%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Aerilate Mega Pinsir Return vs. 120 HP / 248+ Def Alomomola: 412-486 (82.2 - 97%) -- 62.5% chance to OHKO after Stealth Rock
And Mega Pinsir can do this with a lot of other types who don't have pokemon to resist the Flying + Fighting/Ground coverage, as with sticky webs and stealth rocks it can outspeed and ohko a majority of the monotype metagame with very few pokemon switching into its moves.
However I'm not calling for an outright ban, as I still sit on the fence with a possible Mega Pinsir suspect/ban. In my experience using it Mega Pinsir almost felt like Mega Altaria or Mega Charizard in that after a few setup moves and it is ready to sweep teams with a bit of support from its teammates to set up stealth rocks. The main difference is that Mega Pinsir doesn't get a speed boost, and so knowing your calcs when something will out speed but get kod by quick attack is semi more important, but it still personally gave me the mindless click 2 win feel that the newly banned flying megas gave, as opposed to something like Mega Gyarados who feels like it needs it teammates a bit more since it can't actually set up on a majority of the metagame, but that's just my feelings toward Mega Pinsir, who if anything I am leaning towards no ban but I have yet to see all the arguments pro and anti ban.