I've been meaning to bring this up for a while, so I'm glad that it's being considered, because I've found Mega Sableye to be pretty overrated as of late. I was honestly a little surprised when it was promoted to S Rank originally, but I didn't complain because let's be real, Mega Sableye is a really good Pokemon. Plus, the diverse control it had over the hazard game really shook things up and left a huge impact on the existing hazard environment of the metagame at the time. However, as has been discussed multiple times already, the hazard game as already evolved quite nicely to deal with Mega Sableye. More and more people are using hazard setters like Lum Berry Garchomp, Earth Plate Landorus-T, and Clefable, which all have the ability to muscle their way past Mega Sableye and get Stealth Rock up. Even when you're talking about really passive hazard setters like Ferrothorn, it's not a given that Mega Sableye will be able to stop them either. If your opponent leads with Ferrothorn and you don't lead with Sableye or something that can KO Ferrothorn, your opponent is going to get Stealth Rock up regardless of the existence of Mega Sableye on your team. If you lead with Sableye and your opponent leads with something that threatens it (Mega Gardevoir, for instance), then Sableye will be forced out before it can Mega evolve and your opponent might be able to bring Ferrothorn in before you get the chance and set Stealth Rock. I realize that there are a lot of different scenarios that can happen here, but the bottom line is that players have been able to adapt nicely so that they can get Stealth Rock up even against Mega Sableye teams. I think it says a lot when we have a recent Smog article that recommends using a hazard remover alongside Mega Sableye, which is pretty telling for a Pokemon so hyped as a tool of hazard prevention.
I also think Mega Sableye's defensive capabilities have been exaggerated a bit. Base 125 Def and 115 SpD are great; base 50 HP is not, and it means that Mega Sableye's bulk is far less impressive than its high defensive base stats would lead you to believe. Combined with the lack of Leftovers recovery, this means that Mega Sableye is often going to be stretched thin with regards to what it can safely come in on. Piling EVs into one defensive stat and focusing on that side of the defensive spectrum will enable Mega Sableye to take hits really well on one side, but that's also going to leave it open on the other. For instance, you can run max Def and better handle things such as Landorus-T and Terrakion, but you'll be cleanly 2HKOed by things like offensive Heatran and Thundurus. You can run max SpD to handle those two, but now those aforementioned physical attackers are threatening with 2HKOs of their own. And of course, splitting EVs is just going to water down the bulk of a Pokemon who's only taking physical hits about as well as Terrakion and is taking special hits even less effectively. The current spread on the analysis is 252 HP / 132 Def / 124 SpD Careful, and while this spread takes overall hits decently enough, it's still getting threatened with 2HKOs by things like itemless Garchomp's Outrage, Specs Keldeo's Scald, Clefable's unboosted Moonblast, LO Thundurus's Thunderbolt, LO Bisharp's Iron Head (plus those flinches), etc. Even with the attacks that it can escape 2HKOs from, it survives by such small margins that even the slightest bit of chip damage will remove its ability to safely switch in.
Finally, as has been mentioned also, people just know how to prepare for Mega Sableye now. There are plenty of great Fairies out there (which will be even more of an issue for Mega Sableye if Mega Metagross gets banned), things like the Mega Charizards and SubCM Keldeo can threaten it, and some people have even found value in more niche stuff like Mega Houndoom. Stallbreakers like Calm Mind Clefable and SD Gliscor have grown in popularity, and they all have the ability to muscle through stall teams that rely on Mega Sableye to handle traditional stallbreakers. The metagame overall has just adapted to it so well, and this definitely shows when you look at its rapidly declining popularity in SPL. As of Week 5, Sableye only has 6 uses in SPL, and 4 of those came from the first week alone. Now compare this to Keldeo (31 uses), Metagross (22 uses), and Landorus (15 uses). Of course, I realize that usage stats don't tell the whole story, but this is SPL that we're talking about here. These are some of Smogon's best players duking it out in this tournament, and if Sableye has hardly been touched since the first week of SPL, then it probably means something.
Overall, Mega Sableye is still a really good Pokemon. It still has unique control over the hazard game, the Calm Mind sets can still act as a good win condition for some teams, and it still checks a lot of stuff. It's still a nice catch-all for defensive and balanced teams with boundless amounts of utility. However, I just feel that a lot of Mega Sableye's best assets have been exaggerated a bit, and with the way the metagame has adapted to it, I don't feel that it's powerful or influential enough to warrant S Rank any longer.