I think I get what they were going for with M-Slowbro. Not every pokemon that gets a Mega is to make it outright
better. Slowbro afaik, was already UU and OU viable I think, much like Garchomp, Tyranitar or Scizor, it's just meant to give you a different way to play it. Like how most Slowbros in Gen 5 and maybe Gen 6 were the physically defensive utility one (Leftovers with Slack Off, Scald, TWave, Psyshock), and I think people started using 4 atks + Assault Vest, this Mega gives you a different approach to play it, while not completely invalidating it's normal form (much like how Assault Vest Tyranitar and normal Garchomp for it's Speed are still used). It looks like bolstered defense and spA + a crit proofing ability makes it good for Calm Mind, like Blue and Sorbet said, and it still has recovery. Whereas, if they kept Regenerator, it would have so thoroughly outclassed an already good pokemon, that it wouldn't have really gained any new opportunities/playstyles.
This is also better if you're a Slowbro user, as you have a lot more room for mind games: Are you gonna Mega Evolve and begin stacking boosts? Well I'd better switch something in that could end the sweep as soon as p-- oh ****, it was a physically defensive normal slowbro, paralyzed/burned my intended counter
The same vice-versa, keep/send in a status absorber thinking to shrug off the Thunder Wave or Scald, but wind up giving it time to mega evolve and get a defense boost and +1 SpA and SpD, that time wasting a turn that you'll have to switch your absorber out while it goes to +2, or heals with Slack Off, etc.
Just my 2c, I think the devs knew what they were doing. I doubted them about M-Altaria too before I really analyzed their choices.
edit: just checked smogon and the first set I see is CM
Oh well
I still think with the right stat distribution, M-Slowbro could outclass regular slowbro even with the - all things being equal - better ability... Like if it gets +50 Def for 160 for example. It still wouldn't outclass the physical tank set or choice specs sets, as those REALLY rely on Regenerator, whereas you don't really want to switch out your set-up sweeper too much anyway.
