No offense but after reading this I highly doubt you ever properly used Alo because most of this is just not true.I don't know how I feel about mega luvdisc moving into B. It's hard to fit it on any team archetype besides stall/semi-stall because it's so passive. It can threaten status with scald and toxic, but so can every other fat water wall. It can knock off items, but tbh knock off isn't as powerful as it was last gen because most teams carry both a mega and a z-move, which means most teams have multiple switchins. This meta's attackers are too strong that alomomola gets easily overwhelmed outside of stall teams/regen cores that can support it. I think clefable may even be a better wish passer cause it has more defensive utility and although it's HP is far lower, most of the things you'd pass to outside of stall don't have massive HP stats so it is sufficient.
Slowbro I don't think is the best comparison, cause it has instant recovery, regenerator, can get reasonable chip with its SpAtk, gets thunder wave for more utility, and can be run as a wincon with calm mind. It's mega, which sits in B-, can do the same, but swap regenerator for a massive defense stat and a good SpAtk.
Gastro, also in B-, has 2 immunities, a lot of the other same characteristics, and can also function as a wincon with curse.
252 SpA Choice Specs Greninja-Ash Dark Pulse vs. 36 HP / 252+ SpD Alomomola: 253-298 (52.7 - 62%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Hidden power electric, if it were to run it, is also a 2HKO. Hydro needs some chip for the 2HKO. Idk that I'd call that a counter. I'll give you 'check,' because if it switches into shuriken it can force ninja out, and if it switches into hydro from full it can wish-tect stall out the 8pp.
It's only niche over toxapex or bro is wish passing, which isn't a very effective strategy in this very offensive meta (again, outside of stall/semi-stall), I think C+ represents that viability just fine, because it has only one set that works on only specific teams. It would feel out of place to me among the likes of Dragonite, zong, loom, mega gyara, all of which offer more than one specific utility and can find their place on a variety of teams. And it's defensive utility certainly doesn't exceed that of it's fellow C+ inhabitants, Hippo, Quagsire, and AMuk.
Role compression is the name of the game with defensive and utility mons in SM, and alo doesn't compress any roles.
Alo can fit on more than stall and semi-stall, I actually used it on a balanced team and got reqs with it for the Dugtrio suspect. I'd even argue it's better on balanced teams because balance has way more mons that appreciate Wish support while the vast majority of mons of stall teams have recovery of their own, which is why stall is better off running Toxapex. However, Pokemon like Zygarde, Tapu Bulu, Heatran, Magearna, Ferrothorn or Lando-T are all great fits for balanced teams in this meta, have good synergy with Alo and really appreciate Wish support, so I don't see how Wish passing is not a very effective strategy in this meta.
Alo itself may be passive but thanks to Wish + Regenerator it actually maintains momentum pretty well. You can send it in on something that can't do much to Alo and then click Wish the following turn as the opposing mon is forced out and thanks to the Wish you get a risk-free switch into your answer to the opposing switch-in while Alo recovers all the damage thanks to Regen. This is why Clefable is not anywhere near as good as a Wish passer, it needs the Wish to stay healthy itself on top of not being as bulky as Alo is.
Your calcs about Greninja were already adressed above, so I'm not going into that again but I just want to make clear that Alo is indeed a counter to Greninja and that's the main niche it has over mons like Toxapex and Slowbro aside from Wish-passing. Having a full-on counter to Gren is definetely valuable for balanced teams, it saves you the trouble of scouting its coverage and gets rid of the risk of Gren having just the right coverage to destroy your defensive core.
I personally would be in favour of Alo rising to B- because there is no other mon that can fill the niche of countering Gren and Wish passer like it does. It's also a solid check to Scarf Phero and Mega Metagross lacking Thunder Punch and dealing with this amount of (or at least variants of) top metagame threats should be reflected within its ranking and B- seems more appropriate for that than C+.
I also have some replays saved that showcase how well Alo can do so this isn't just theorymons: