Celticpride your list is neither complete nor entirely correct and contains a variety of niche/specialized Pokemon. For instance, Mega Gardevoir does not 6-0 Sableye stall unless you're talking about a bad team that doesn't include Jirachi, and even if you include Pursuit support, it's not like stall can't check Mega Gardevoir via wearing it down with hazard pressure, Chansey, and Brave Bird Skarmory; Mega Gardevoir also can't switch into Mega Sableye because it's weak to Shadow Ball and dislikes taking Knock Off. Mega Heracross is practically unviable in the metagame as it stands because there is practically no reason to use it over other top Fighting-types, especially Mega Lopunny, and practically no reason to use it for your Mega slot when there are so many choices that fare better against offense; Sableye actually kind of checks it with Prankster Will-O-Wisp and regardless stall is well-prepared for Mega Heracross with Unaware Clefable/Gliscor being crucial team members. Mega Charizard X being a check actually completely depends on the variant because while it does deal with CM versions effectively, utility versions with Foul Play just kill it. Landorus-I can't even break Mega Sableye if it has set up a Calm Mind already and fears a Knock Off; it typically can't even break SDef Mega Sableye in a 1v1 scenario. Thundurus-I doesn't even beat CM Mega Sableye 1v1 unless it has Nasty Plot which does win 1v1, but it has no ability to beat Mega Sableye if it has already set up a Calm Mind; it would have to sac itself to get off the significant damage on Mega Sableye, and furthermore Mega Sableye can just chip off 50% with +1 SBall or burn it and then switch out.
Mega Absol is a niche Pokemon as-is and loses to utility Foul Play versions of Mega Sableye pretty handily as Foul Play 2HKOes on the switch and it loses 1v1 if it tries to set up an SD. Mega Pinsir just gets burned and then you can switch out your Mega Pinsir answer, while Mega Pinsir is rendered effectively useless. I don't know how Victini is a 'check to it' when the best it can do is revenge kill with CB V-create and though it obviously gets off the damage needed it still loses 1v1, missing the KO with V-create and dying to any attack after the drops if it has taken SR damage or just outright being KOed by Foul Play; other variants of Victini can't even get off enough damage to weaken it into a sufficient range to where it is no longer much of a threat. Against fully physically defensive variants, Diggersby falters because LO EQ doesn't even 2HKO with the first hit + a burnt hit, so if it's 252/252+ Mega Sableye can just burn it, recover up if Diggersby decided to attack, and switch out. Crawdaunt lost almost all of its viability in the transition to ORAS with so many Pokemon that can threaten it/beat it being everywhere in the metagame, Mega Altaria not being the least of those, and standard Jolly LO doesn't even 2HKO max Def variants with Crabhammer + burnt Crabhammer while Adamant LO still doesn't even have a 50% shot to do so and is outright inferior to Jolly on the whole.
Obviously a lot of this depends on the variant including EVs and whether it's CM or a utility Knock Off/Foul Play variant, but all of these versions of Mega Sableye are very common and thus it is difficult to regard a lot of those Pokemon as answers to it. The Pokemon that can actually switch into Mega Sableye and beat it without being enormously crippled that are on your list are CM/Unaware Clefable (yes, CM Mega Sableye beats standard SR versions of Clefable), Mega Charizard Y, Sylveon (which is kind of garbage), Mega Diancie, Mega Altaria, Mega Gardevoir (a stretch because it can't switch in), CB Azumarill (this is a bit of a stretch because it's not really a switch-in), and Mega Houndoom. In your list, you have neglected to include SD Gliscor, Mega Lopunny (also not a switch-in at all), and TG RD Manaphy. For a Pokemon that does so well against the majority of the metagame, especially when half this list is composed of Megas and some of the best answers aren't even real switch-ins, this is a limited set of answers.
A Mega Sableye suspect certainly isn't 'nowhere near the top of the list', especially seeing as it is perhaps the #1 Pokemon being complained about by experienced players of ORAS OU. It's easy to see why because Mega Sableye checks/counters at least over half of the tier with any given set and around 3/4 of the tier if you account for all of its possible variants WHILE being a devastating bulky sweeper with its CM set that has a limited list of counters and even checks AND making common defensive Pokemon such as Ferrothorn, Skarmory, Chansey, Celebi, Jirachi, Slowbro, etc into liabilities AND being the most effective form of hazard control the tier has ever seen. Whether Mega Sableye is actually banworthy or not is definitely debatable, but you can't argue that there are no points against it and that it isn't one of the top choices for a suspect in the near future.