Victini is a really good mon. I'm not going to argue that it should leave S as I haven't played Mix and Mega a lot, but I'm going to argue that Primal groudon is better than a pityful A- rank.
Primal Groudon has almost no downside
I mean... Look at it. It has one weakness in ground and it's not like there's a lot of ground pokemon that can OHKO this thing either. Never mind the fact that the ones that do are easily outspeed and OHKOd. It's one weakness and immunity to water and electric allows it to switch into a plethora of mons. I mean, in Ubers this thing has made the whole viability rankings revolve around it, if you give Primal groudon a free switch in you're essentially doomed in terms of viability. Rotom-W? Gone. Kyogre? Gone. Kyogre went from being S rank to what... B? C? That's saying something. It's not like anything in this metagame can even be compared to primal Groudon, maybe Hippowdown? No, because you have to remember that there isn't a downside to Primal groudon. Great HP, Breathtaking Attack, Baffling defence, Astonishing Special attack and above average speed and special defence.
Primal Groudon has no counters
It's unmatched ability to go mixed and with access to a plethora of set up moves leaves this mon uncounterable. Giratina? Nop, 80% from +2 Preciples blades. Lugia? Stone edge breaks its ability to recover up, as well as fearing Preciples blades if it chooses to roost, never mind the fact that Lugia nor Giratina can do anything to this monster. Uninvested Draco meteor from giratina doesn't even 2HKO. And don't try to mention Giratina-O as it has a chanced to get OHKOd by +2 Dragon claw. And yes, these pokemon and others can use Mega stones. But really, fire, ground + rock is unresisted and at +2 nothing takes Primal Groudon on. Quagsire takes 60% from Preciples blades even if its fully invested, that is breathtaking.
Primal Groudon fits on every single play style, there is not a single reason to not use this monster
There's a reason it's on 50% of teams in Ubers. There is virtually no reason to not use Primal Groudon. It fits on stall with its support and defensive abilities. Balance loves a water and fire spam switch in. Hyper offence has nothing that can do a duel dancing role better Primal Groudon. Why would you not want to use Primal groudon? It has no opportunity cost as every team should have a ground switch in. It can be your rocker, your t-waver, your sweeper, your wallbreaker!
I can bicker on about Primal Groudon for ages but essentially? Primal groudon loses nothing from this metagame, very few things can stand up to it and it still has the highest BST out of any pokemon, bar the MMXY and Kyogre. It can preform a large variation of roles that can fit on every single team. It's not an A- pokemon that's absurd.