I feel like the introduction of Volcanion will hurt the viability of certain Pokemon. While it is fairly new to the meta, it still takes a hot, steaming dump (pun not intended) on a good number of OU Waters, and a good number of Fire types. It's morely annoying when firing off its own brand of Scald on steroids, creating 50/50 situations of fucking you up similar to when we had Aegislash.
I agree with you that Volcanion hurts the viability of certain Pokemon, and because of Water Absorb, it can laugh off the protests of several Pokemon. However, when you think about it, Volcanion cannot really do much back against Water-Types. Let's take Manaphy for example. It switches into a +3 Scald and gains a chunk of HP back. It laughs off Ice Beam, too. Now, Volcanion's movepool is somewhat shallow and only really consists of a buffed-up Scald, Fire Blast/thrower, Sludge Bomb, E-Power and a HP of your choice, none of which can hit any of the premier Water-types for substantial damage. It isn't very uncommon to have Manaphy run a coverage move such as Psychic or Energy Ball, and those can definitely break past Volcanion:
+3 252 SpA Manaphy Energy Ball vs. 252 HP / 0 SpD Volcanion: 223-263 (61.2 - 72.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+3 252 SpA Manaphy Psychic vs. 252 HP / 0 SpD Volcanion: 223-263 (61.2 - 72.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Factor in possibilities that Volcanion carries a Life Orb, and Manaphy can set up another Tail Glow, stomach Sludge Bomb or Earth Power, then use another coverage move.
There's also Keldeo. Keldeo isn't weak to any of Volcanion's usual moves and Calm Mind in it's face, or use Secret Sword. Secret Sword probably won't do deadly damage considering Volcanion's above average bulk, but on the other hand, Keldeo also now has a setup fodder.
The only OU Water I can think of that actually loses to Volcanion is Azumarill, which is because of it's resistance to both of it's STABs, and ability to use Sludge Bomb.
At most, Volcanion is a good pivot into Water-Types when versing Waters. It's definitely a good Pokemon, but I think you're kind of overrating the fact that it "takes a hot, steaming dump on a good number of OU Waters".