Okay so this is in fact very impressive. I can see him in B+, maybe A. Do you find that regenerator is always enough to keep him going throughout a match? He doesn't look too difficult to wear down, but I know from experience that regenerator is deceptively effective.
Nah, his job as a defensive pivot allows him to avoid it. He's generally in, take a hit/give a hit and out. I have chansey around but I might pass one wish to him in a 90+ turn game. Unless the opponent is running full pressure, there are just too many easy in/out switches for him to get worn down easily.
I think that Slowbro should rise to B+, but not A. Its not outclassed by any pokemon and is a great part to a defensive core with mega venesaur. I believe you, I've played against your stall team with it. However it has a few problems of its own that prevent it from functioning as well as it would like and it can't be thrown on any team. Its special defense is somewhat lackluster, and an assault vest prevents it from using healing on the spot and using thunder wave. It can be pursuit trapped by Band Tyranitar, and actually as a chance for OHKO. It can also be walled by pokemon such as mega venesaur (if they have some sort of status support) or celebi. I think that B+ is still pretty good (up there with skarmory) and that it definitely is viable in today's meta. Its certainly as good as Goodra, another B+ pokemon.
Obviously, the majority of the meta doesn't include bandtar (What is his usage this gen anyways...?). Mega venu isn't coming in, as psyshock is still an option.
(for reference)
0 SpA Slowbro Psyshock vs. 252 HP / 0 Def Mega Venusaur: 146-174 (40.1 - 47.8%) -- guaranteed 3HKO
4 SpA Mega Venusaur Giga Drain vs. 252 HP / 36 SpD Assault Vest Slowbro: 150-176 (38 - 44.6%) -- guaranteed 3HKO
Yeah, venu wins, but he isn't coming in on that. This is just the set I run. The physically defensive support set is viable as well, but loses a bit of the pivot mentality. Does gain the twave support (should you use it) but I have just found this AV set to be incredible. Preference on AV, though. Remember, it isn't that he CAN'T run another set beside AV, it's just that I don't and so I'm just arguing what I see to be the better one.