Ok I guess if you're looking at stall as a whole now then you have to look at all the elements of it in order to find out what the busted part of it is. From what I can see, they all contain the following elements:
1) Chansey
2) Defog (usually Skarm)
3) Unaware (Clef/Quag)
4) Regenerator (Toxapex)
5) Sab
6) Trapping (Dugtrio)
The first three parts are pretty uninteresting because they've been around since the start of gen6 and never caused any real problems. The last 3 have all been introduced in SM, unbanned, and buffed respectively. It's actually tough to isolate exactly what to ban, but logically, it's one of Dugtrio, Toxapex, or Sableye. At the start of this gen I decided I was going to try and vote ban as little as possible. In the first suspect I felt there wasn't really any adaptation or option other than to ban Genesect, but this suspect we have to decide what part is broken, if any at all.
I've already typed up a ton of posts about
Sableye, but I guess I'm going to have to type up another paragraph, and actually I think it's a little different in SM. Every time you load up a Sableye team, it does take skill out of the game, but ultimately it's a team preview gamble about whether you see opposing breakers you're weak to, or not. As Blunderr mentioned in the OP, it's not as good at mitigating damage in terms of preventing hazards as it was in ORAS, but also to bear in mind is that if you look at the number of new breakers introduced, the gamble's much worse this gen. By that I mean we've got Lele introduced, Hoopa unbanned, which are the two big factors, but there's also stuff like Xurkitree if you want, Tapu Fini for the breaker set, Z move breaker sets, etc. I literally can't name all the new powerful threats we've got. The issue back in ORAS was that there was no guarantee you had a breaker that was super threatening to the opposing team, but with this many options, the likelihood of that happening is way bigger.
My conclusion for this pokemon is that Sableye generally makes games revolve around 1 or 2 members on your team which are breakers, but when the breakers are so splashable and so good, maybe that's ok.
Toxapex obviously walls a ton of stuff, and it makes the stall core way more resilient that it would otherwise be. The typing is definitely interesting, but it has its drawbacks for sure. Ie it is weak to ground, it is weak to psychic instead of resisting it like Slowbro, so it can't wall Metagross, and while it has haze, it still has to take the boosted hit to get the haze off. While it beefs up the stall core, it doesn't really make it invincible. Also because its scalds are so weak, you can kinda just get around it by using substitute sets, like Buzzwole can run a sub lefties set. I mean it's a factor as to why stall got better, but is it the broken part? I doubt it.
So the last potential issue is
Dugtrio, which I think is likely to be the greatest offender. The reason why is because when I look at the 5 mon core of Sableye, Toxapex, Chansey, Skarmory, and Clef, I see a core that's weak to a lot of stuff. It gets blown apart by Lele, Hoopa, Xurkitree, and Magma Storm Heatran can be threatening. However, what Dugtrio can do, is revenge all of those effectively, and many more breakers.
Last gen I wrote this:
http://www.smogon.com/forums/threads/arena-trap-dugtrio.3582873/#post-7000913
While I stand by what I said back then, Dugtrio has received a significant buff this gen, in 20 base attack. That's 40 additional attack on a pokemon balanced by the fact that its amazing trapping ability was offset by its weak attack. Dugtrio plays a key role in these teams by basically being a near omni-check. By that I mean it's faster than basically every breaker, and even if you are weak to a particular threat, you can get off a guaranteed earthquake after it gets just 1 kill, but more likely you'll get off 2 earthquakes if you're smart and keep hazards off the field. We all know from shadow tag that powerful trapping can be devastating when paired with stall, and it seems that the additional attack has made Dugtrio's trapping powerful enough to deal strong neutral damage rather than having to rely on the trapped pokemon being weak to eq to beat it. This means that its scope of trapping is way greater than it was before.
Anyway, the effect of Dugtrio is that you can't rely on just 1 breaker to beat stall, you have to stack them. This might not seem like the worst thing, but it does mean you're forced into a very specific style of building to beat these teams, ie carrying at least 2 breakers they're weak to instead of 1, which isn't good when you have a lot of other threats to account for. Like I said earlier, I want to ban as little as possible, and so I'd like more replays before making a firm decision.
However, one thing I firmly believe in tiering, is particularly when we have this many threats to account for, we shouldn't let something which takes skill out of the game by relying on matchup exist in the tier. It's much more difficult to account for matchup based styles in newer gens than past gens when building, because you are way more strapped for teamslots. I struggled getting suspect reqs for Genesect with conventional teams, but when I switched to SabDug stall halfway through laddering, I could pick up a better winrate despite not really thinking about my plays like I had to before, or being particularly good at SM in general.
In conclusion I think that
Dugtrio is the threat we should be looking at, and if it continues in the same vein as it has done thus far (perhaps there's more counterplay to these teams which hasn't yet been discovered), then removing it might be the correct decision.