Fairy Stab still isn't that common in Sun and Moon. Tapu Koko's thunderbolt does similar damage to dazzling gleam while holding magnet under electric terrain, doing just over half to the max Special defensive set. Dazzling gleam with life orb does 58.8-70%. Hence it's quite risky to switch it onto a Mega Sableye if he's carrying toxic (which I recommend), as he can recover stall you. Noticeably Mega Sableye is paired with chansey (even increasingly outside stall) which handles Koko pretty well. Tapu Lele is often specs or scarfed and of course if locked into psychic is useless against it and again handled by chansey. Mega Sableye can usually recover stall on fini too until misty terrain ends, as taunt is useless as it's bounced back, the defensive set doing 42.1%-50% to Mega Sableye. Magearna is a problem but handled by chansey pretty well. The pokemon where it has the worst match up, i.e. can't do anything+doesn't have a safe switch in with chansey/steel type for are Mega Lopunny, Mega Gardevoir and Mega Mawile. And only Mawile is seems up there in viability atm.
It think it is as splashable mon as pokemon in tier A as in my opinion like it was last gen, not hard to fit him on a team, pairs well often with a steel type to cover the only fairy weakness. With Hazards (and thus toxapex, spikes greninja, ferrothorn), status,Dugtrio, leech seed (particularly on ferrothorn and Celesteela) running rampant, provides arguably the best check. Also what I've found is having a mega sableye on the team reduces the need for a defogger, allowing greater freedom in teambuilding, with good defoggers scarce in OU (Tapu fini being the best IMO which has lost viability of late, and Latios always a risky one with pursuit trap and giving a free switch in to magearna which has risen in usage).
There are many reasons to run it over it's competition in rank A. Tangrowth assault vest, it's best set, takes special attacks only marginally better than specially defensive Mega Sableye but has more weaknesses, no access to recovery (though ofc regenerator offsets that a bit) or status when using assault vest. Toxapex weaknesses to commonly run stab in electric and ground, is trapped by dugtrio, though is a superior hazard setter. Ferrothorn really good at taking hits with incredible typing, but lacks reliable recovery (leech seed isn't too reliable). Sableye can function as a suitable alternative (or it can pair with well with all those pokemon, particularly toxapex and ferrothorn), I don't think it compares badly with pokemon that do a similar in the A tier.
Saying Mega Sableye is splashable on anything outside of stall is blatantly false. I cannot honestly think of a time outside of stall (something I rarely use) that I have even considered running it, and I've not seen a single Mega Sableye outside of stall in higher tier play so I'm clearly not alone on that. Mega Sableye's very passive and forfeits huge amounts of momentum as a result (something you forgot to mention), meaning it's impractical to fit on offense, and on balance/more defensive (but not quite stall) teams, there are usually better defensive megas they can run (stuff like Mega Sciz, Mega Venu, Mega Slowbro or wallbreakers such as MHera or MMaw), with practically no reason to run Mega Sab over any of these, except for really niche cases. Not sure why you're comparing Mega Sab to any of the A mon pivots considering they actually have resistances (poison lol). The only playstyle where it's splashable is stall, because there are far better megas on every other playstyle. Furthermore, most of the things it answers can be beaten by better, more splashable Pokemon (Mew for example).
The only reason Sab's ranked so highly is because of it's impact on stall and Mega Sab providing so much support to it by helping keep hazards off the field, helping (read: not 100% guaranteed) prevent teams from just doubling against stall to get hazard chip and deflect Taunts if need be, and it's current rank reflects it adequately. Also why would you run max Sp.Def? What does it do except make you more prone to KokoMMedi and let Defensive Lando weaken you so much more? What do you mean "good defoggers [are] scarce in OU" when Mew and Zapdos exist? It's also hardly a suitable alternative to hazard control because most teams are already prepared to get hazards up vs Mega Sableye (stuff like SD SR Lando, SR Clef, SD SR Chomp, Heatran etc or) due to wanting to not lose to or at least want to be able to pressure stall.
I'm wrong in saying that it's handled by chansey, however the stallbreaker set isn't too common in my opinion really, though I agree it exists.Pairing with a steel type instead makes it harder for Lele.
Heatran doesn't get rocks up on the specially defensive set, sure it resists toxic and will-o-wisp but can't be taunted, and can recover on heatran. Yes it can eventually be forced out by risk of a lava plume burn or running out of protects. As for trapping Sableye with magma storm, it only does 41.7% max with timid air max sp.A heatran. Landorus fears the burn by will-o-wisp so staying in isn't too desirable. It's not a momentum sink when the opponent has to fear toxic, will-o-wisp on switch in. And physical attackers find it difficult to set up due to existence of foul play too. It doesn't do anything to clefable, which hasn't changed since ORAS. The best set IMO (and I'm sure I'll get bashed for this) is Will-o-wisp, Toxic, Foul Play, Recover, partly because it threatens more pokemon, and is less momentum draining.
Statistically it is one of the best special or physical walls, with the appropriate movesets, resistances, and just one weakness. I think it's a bit strange to say it's impractical outside stall, when it has been utilised effectively in the past and still can be. There's just few reasons for that. It's not as good as ORAS where eventually banned to ubers but it's still A level worthy. An extra defogger is appreciated, but Sableye finds use in teams which don't want to include a defogger, itself isn't weak to rocks. It is definitely more difficult to stack hazards on the field with a Mega Sableye at the opposite end. It's not a suitable replacement for a defogger say if you're using something like volcarona on your team, you need a more reliable, but in a non SR-weak team it finds it can be sufficient.
Only compared Tangrowth as it was one of the pokemon in Tier A. Quite frankly Sableye doesn't have many pokemon that can do quite like what it can do,which while a personal preference to some degree, I tend to rate Pokemon a little higher than can bring something unique.
I've probably gone a bit overboard in listing its advantages. Its' main reasons for rising is the rise of dugtrio, hazard stacking, continued ability on stall, and finally it actually works pretty well against stall too.
Even if it only has weakness, it's got almost no relevant resistances (offensive poison types LOL), the only relevant one being Fighting (EDIT: And Psychic, but lots of psychics have ways of hitting it anyways), and MMedi in ETerrain easily 2hkos with TPunch, especially with that god awful max sp.def spread. Mega Lop, the only relevant offensive Normal type I can think of off the top of my head, spanks it with Scrappy Return or HJK anyways. Combine it with the fact it has no passive recovery and it's relatively easy to wear down. Tangrowth is far more physically bulky to boot and has a better defensive typing due to actually having relevant resistances (water, electric) and has passive recovery in Regenerator. It's a far superior pivot to Mega Sableye. Heatran gets rocks up vs it by either burning with Plume or chipping it with Magma Storm secondary damage and forcing Sab to Recover nonstop, eventually PP draining it or allowing a wallbreaker to get in on the recover/protect and immensely pressure it, enabling Heatran to beat it later.
It's nowhere near "one of the best walls" lol, the fact you said that is a straight up insult to Toxapex and Tangrowth among others. They are bulkier (far more so in Pex's case), and have much better defensive typings that mean they actually check things without relying on raw bulk. Normal only has one weakness but you don't see any defensive normals outside of Chansey. Oh yeah and 50/125/115 defenses isn't even that amazing anymore due to Power Creep(TM)
Nothing has changed for Mega Sab to go up a rank, and in fact it's not even guaranteed on stall anymore, pointing more to MSab having gotten worse, not better. Duggy+MSab is far from new, it existed during ORAS post STag ban. Duggy's the only mon I'd call being on all teams and even then there are people trying to "innovate" with Dugtrioless stall, most teams usually notably suffering from the opportunity cost of not running Duggy.
Keep Mega Sableye A-
