Anyone who uses Mega Diancie in Rock mono and a walrein WITH Frost Breath in an Ice mono aren't going far with those teams. Rock NEEDS Mega Aggron and Choice Specs Walrein....... I'll just let you figure out how bad that is of an option.
Mega Sableye is an auto win against any electric team, Mega Ampharos' base 165 special attack can 2HKO Mega Sableye (and thats the highest special attack stat electric gets), but if even one calm mind is up, a crit is needed to win. Now the other options are hitting it with fairy moves from Dedenne (only electric Pokemon with Play Rough) or Lanturn with dazzling gleam. The argument of don't let it set up doesn't matter here cause alot of the time it uses calm mind with the prankster ability still in use so the first boost can't be stopped.
252+ SpA Mold Breaker Mega Ampharos Thunderbolt vs. +1 252 HP / 0 SpD Mega Sableye: 115-136 (37.8 - 44.7%) -- guaranteed 3HKO
252+ Atk Dedenne Play Rough vs. 252 HP / 252+ Def Mega Sableye: 120-144 (39.4 - 47.3%) -- guaranteed 3HKO
252+ SpA Choice Specs Lanturn Dazzling Gleam vs. +1 252 HP / 0 SpD Mega Sableye: 120-142 (39.4 - 46.7%) -- guaranteed 3HKO
The only way I see Electric managing to beat just Sableye is with Nasty Plot Thundurus, but with Stealth Rock damage Thundurus is 2HKO'd by +1 Shadow Ball, so it would require amazing predicts and luck to get Thundurus set up enough to kill Mega Sableye, if it is even possible. Now thats pretty much 3 types that can't handle one Pokemon, and I'm pretty sure Good Old Smogon Bird got quick banned for doing something like that
Sorry to come in on this but Talonflame is on a strictly different level than Mega Sableye is, which is why it got quick banned. Talonflame has the ability to deal with Grass-, Ice-, Bug-, Fighting-, and Steel-types. That's 5 compared to 3. It doesn't just deal with those Monotypes, but also causes trouble to many other types in the game. Also with Gale Wings, Talonflame also causes a lot of trouble for other Monotype teams, giving it much needed KOs or puncture holes through a team with Choice Band. With the use of Swords Dance you can predict Sucker Punches and switches which will give a free turn to set up. Also with Will-O-Wisp, it can help provide support for its fellow teammates. With the Speed, power, support, and priority it carries, Talonflame was much harder to deal with than Mega Sableye and I can't tell how much of a pain it is.
I don't necessarily see why you should run Mega Aggron over Mega Diancie. Mega Diancie is literally a nice counter to Mega Sableye because it resists its Dark-type STAB moves. Also its Fairy typing and access to Moonblast and Calm Mind gives Mega Diancie the upper hand. All Mega Sableye can do is simply Calm Mind in hopes of defeating Mega Diancie with one hit from its Dark-type moves. Mega Sableye would even need to set up multiple Calm Minds to prepare itself for Mega Diancie. Also, losing out on Prankster and gaining Magic Bounce hurts it against Mega Diancie. All the Speed loss it gets. Will-O-Wisp doesn't affect Mega Diancie because of Magic Bounce. Mega Aggron on the other hand can't do anything to Mega Sableye other than keep it from setting up Calm Mind with Dragon Tail. Besides, Mega Aggron lacks recovery options and Mega Sableye can just simply burn it. Who even uses Dedenne? As far as I know Dedenne still can't do crap to Mega Sableye, as it still has trouble even 3HKOing it with STAB Play Rough. Mega Sableye can also burn it with WoW and Recover from there. Dazzling Gleam is a much more prominent option because it actually 2HKOes Mega Sableye or 3HKOes it if it didn't set up with Calm Mind already. Also Dedenne is pretty much an irrelevant mon because its poor bulk just gives you a wasted slot on your Electric team.
I wouldn't forget about Choice Specs Thunderbolt Magnezone because at the same time Mega Sableye will also being asking for a crit and can't properly set up without it being gone. Luxray is also considered a nice check because of Guts and it can carry Choice Band too. This is given the fact that we're 6v6 theorymoning and that Mega Sableye isn't at full HP either. Sub + Calm Mind Raikou is also considered a nice check to Mega Sableye too.
Rock needs Mega Aggron because it is their best check to Ground and Fighting Monos, since most Pokemon in those types are physcially based, Filter Mega Aggron is by far the superior mega 'in terms of beating their weaknesses' I'll rephrase it like that because that is true. Even with Mega Diancie against Fighting, I still believe that Aggron is a better fit because most fighting teams I've seen only have Keldeo as the special attacker leading to them getting walled easily. Aggron also stops you getting swept from Scizor in Bug mono, which in my eyes is much more important than beating maybe 2 Pokemon with Diancie and then just losing, but hey, opinions are opinions.
I'll admit I haven't seen much about Ice because it is such a poor type, so I'll happily take back what I said there
As with the electric stats, I used Lanturn and Dedenne simply because they are the only two electric pokemon that get fairy moves, I should have stated that I knew they weren't viable. And Toxic Mega Ampharos, while its an okay check, I would argue that his movepool needs different moves to toxic, simply because in electric teams they need support elsewhere. Running a move simply to check one Pokemon has been the reason some have been banned before, maybe not in this tier but in other tiers.
Also sorry if it looks like I'm talking in a harsh tone so to say, but this is how I normally type and I'm not trying to undermine anyone here or their opinions, I'm simply stating mine and why I think Mega Sableye should be banned in this current meta of Monotype
Mega Diancie can help deal with Fighting-types too. Even though it does need to watch out for Calm Mind Keldeo, Endeavor Infernape, (given that you don't have any hazards set up on the opponent's side of the field yet), Choice Scarf Terrakion Iron Head along with Cobalion. Mega Aeroactyl can as well being part flying and having excellent Speed making it a check to the Mono. Also I don't see how Mega Aggron is Rock's best check to Ground because Life Orb Landorus can just simply OHKO Mega Aggron even with the Special Defense invested along with the ability Filter. Nidoking would also prove to be an issue as well. Despite Mega Aggron having excellent defensive bulk, it'll still have problems with Pokemon such as Gastrodon, Landorus, Nidoking, and Seismitoad. Gastrodon and Seismitoad running Scald and burning Mega Aggron. So I wouldn't disregard these Pokemon either. Also Mega Aggron's Thunder Wave support makes it quite useless against this mono. In my opinion, Rock's best bet in beating Ground-types is Omastar because of Shell Smash. Also I saw that Omastar runs Hidden Power Grass pretty often as well.
Toxic on Mega Amphy is pretty much needed to get more status off. Electric has too many Special Attackers which Mega Sableye can set up on. Also it helps in getting rid of threats too besides it. I run Toxic on Mega Amphy because in a way it helps forces switches. What makes Mega Amphy so good is because of its bulk, Dragon / Electric typing, Special Attack, Mold Breaker, which helps to bypass many abilities such as Skarmory's Sturdy, Magic Bounce Pokemon, and support moves such as Heal Bell. I would like to see what you think other support should lie elsewhere. We're not really talking about one move to run that a Pokemon is still banned. We're talking about ways that Mega Sableye can be checked or countered by.
Also you might want to please expand on your reasoning as to why Mega Sableye should be banned besides Electric having only Nasty Plot Life Orb Thundurus to check it.