So it will likely still be better to carry Heatran?
Depends. Are you Talonflame weak? Then Heatran's your man. Do you have an Azumarill problem? Toss a Heatran in front of that puppy and tell me how that works out for you.
Yes. Volcanion is being massively overrated. Steam Eruption is pretty great, but it doesn't make up for the bad speed, sub-par typing, limited movepool, and above average at best offenses. It's best sets seem to revolve around just spamming Steam Eruption. I'm sure someone will find some "usable in OU" niche but I don't think this thing will ever reach "literally every team can find use for this" OU standard levels like Heatran did.
I don't think anyone's saying that Volcanion will be an OU staple on Heatran's level, but you're definitely exaggerating some of its flaws. I wouldn't say that Volcanion's typing is sub-par. In fact, as long as you can avoid Stealth Rock, it's quite good when factoring in Water Absorb. You trade 3 weaknesses for 5 resistances (3 of which are 4x resistances) and an immunity. I'd say that's a good trade. Sure, it has a shallow movepool, but it's not like Heatran's is much better. The only notable coverage moves Heatran gets that Volcanion doesn't are Dragon Pulse and Earth Power (and who knows, maybe Volcanion will get one or both of those from tutors in the next games), while Volcanion has STAB Water attacks and Sludge Bomb/Wave to smack Azumarill around with. Also, "above average at best offenses?" I'd say that 110 / 130 offenses are pretty great. Not many OU viable non-Mega Pokemon can boast the same combined offenses (pretty much just Kyurem-B, Aegislash, and the genies). The 130 base SpA stat alone is impressive enough, about on the level of things like Latios, Terrakion, and Keldeo. Heck, the fact that Volcanion is much more comfortable running a Modest nature than those guys makes it capable of doing even more damage.
I honestly don't think Volcanion will be top OU (probably low-mid OU at best), but let's not make it sound worse than it really is.
Also:
252 SpA Choice Specs Keldeo Scald vs. 0 HP / 0 SpD Mew: 195-231 (57.1 - 67.7%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Volcanion Hydro Pump vs. 0 HP / 0 SpD Mew: 198-234 (58 - 68.6%) -- guaranteed 2HKO after Leftovers recovery
Volcanion's unboosted Steam Eruption is hitting harder than Specs Keldeo's Scald. Be afraid. Be very afraid.