When 6 / 10 mons in S and A+ are weak to Earthquake (Greninja counts as weak to Earthquake tbh) yes, I do think a weakness to it is a small matter, same with a weakness to any type because no one expects any singular mon to 1v1 the entire meta. This is especially true with Volcanian because literally no Earthquake users can switch in to it.
That's where you kind of fall apart.
Volcanion has great coverage and great STABs. Let's get that out of the way first. That's true, yes? But it can only use one move a turn. You know it's running Steam Eruption, because why the hell wouldn't you, you know it's running a Fire move, and you know the last two are user preference (Sludge Bomb for fairies not named Magearna, Earth Power for Heatran switchins/Toxapex/something else weak to ground, Grassium-Z for the Choice bluff with Solar Beam, maybe HP Electric for Mantine).
Okay, but it can still only use one move a turn. It's not going to be able to use Fire Blast and Steam Eruption at once. Specially defensive Toxapex variants easily take either STAB and take an Earth Power moderately well (Mainly because of Regenerator) in case of a mistake, the blobs don't care, and Tapu Bulu can switch in on any move that ISN'T a fire move and go to town (This is assuming that Tapu Bulu outspeed the Volcanion) and... Oh yeah, Duggy. Whatever, everything grounded is technically weak to Duggy.
What I'm saying is that there are clear paths to counterplay with Volcanion, and most of them are pretty easy and common. One of them is an entry hazard.
But okay, let's pretend for a second that Volcanion is a completely free switch for all the landos, and the garchomp, and the excadrills, and the metagrosses, etc. etc. What can I do? I pair it with lando-t, or celesteela, or buzzwole, and so on because that's what you do when you're teambuilding, you cover types other members are weak to. Same with hazards: if I'm worried the defensive capabilities of a mon will be compromised with stealth rock, I'm gonna run hazard support. The only criticism it gets here is that it doesn't pair well with Tapu Fini, but there's still Zapdos, Latios, and Excadrill (inb4 stacking weaknesses is literally suicide) who all pair with it just fine.
Having to run hazard removal as a necessity is a burden. I won't say that it's a bad thing because hazards always a bitch to deal with, but trying to brush off the "I lose 25% of my HP every switch" as "oh I can just run hazard removal" is trying to sugarcoat the burden on teambuilding. Zapdos and Excadrill are good mons in this meta (I can't say the same for Latias because of how many important weaknesses she's carrying) but Zapdos itself has a rocks weakness and Excadrill is uncomfortably doubling up. I mean, Tapu Fini also works fine with Volcanion (As long as your team also has other things to deal with that stupid rooster), it's just that Tapu Fini doesn't really do much besides Taunting and Defogging.
You've made no arguments except that it has weaknesses (it has three, two of which are covered by type immunities) and that it's weak to stealth rock. You said it can't OHKO Manaphy at all when I literally just mentioned is best set, Bloom Doom, and you had to look up what moves it gets.
Also, since when did being weak to Stealth Rock
not count as an argument? Kyurem-B is solidly OU even though it's faster than Volcanion, still has two great STABs, and has a higher BST- in fact, it shares a lot of similarities, including a massive attacking stat and inability to be properly walled by the vast majority of OU, with Volcanion. (Volcanion probably has a better movepool, though.) So what makes it acceptable in OU? I don't know, vulnerability to all three hazards but Stealth Rock in particular?
Also, you said Manaphy could be
checked by Volcanion. Checking usually means switched in after your opponent plays it correctly and then forcing a switch/shoving it out through sheer damage or exposing a weakness, correct? Yeah, uh, those damage calcs show that that's a fantasy, and I'm pretty sure that unless you managed to get Sticky Web out on the field first, Volcanion runs the risk of being OHKO'd.
I'm fairly confident you've never used or faced Volcanion much at all.
I did. It failed miserably. I'll end it at that. (It wasn't running Earth Power somehow so I forgot about the Earth Power move tutor.)
You realise that literally just confirms that it's not a flying, steel or poison type, right? Looking through the tier list, over half the ou tier fits the exact same description, I'm not sure why this is the crux of your entire argument.
There's a difference between "vulnerable to every entry hazard" and "loses 25% of HP every time I switch in to the most common entry hazard unless I always have my Defogger on deck." The Charizards circumvent this problem by being very strong (Or Charizard-Y just starts things off as a lead), Volcarona is an amazing setup sweeper when done right, and Alolawak has three immunities and access to the game's strongest vanilla Flare Blitz with a Thick Club. Tornadus has Regenerator, Thundurus has Prankster and great stats all around in a great distribution, and Kyurem-B has no real defensive counter in OU- but Volcanion just doesn't offer comparable reward for losing 25% of its HP on every switch. That's a shame too, because it has such good defensive stats and, excluding the rock weakness, reasonably bypassable weaknesses. And don't say "B-b-but mah super great dual STABs," Volcanion can only use one move per turn like the rest of the Pokedex and there's a really good bulky water introduced this generation that giggles at both of the aforementioned STABs.
252 Atk Dugtrio Earthquake vs. 4 HP / 0 Def Volcanion: 236-278 (78.1 - 92%) -- guaranteed OHKO after Stealth Rock
252 SpA Heatran Earth Power vs. 4 HP / 0 SpD Volcanion: 214-254 (70.8 - 84.1%) -- 68.8% chance to OHKO after Stealth Rock
252 Atk Mold Breaker Gyarados-Mega Earthquake vs. 4 HP / 0 Def Volcanion: 214-252 (70.8 - 83.4%) -- 62.5% chance to OHKO after Stealth Rock
+3 252 SpA Manaphy Energy Ball vs. 4 HP / 0 SpD Volcanion: 223-263 (73.8 - 87%) -- 87.5% chance to OHKO after Stealth Rock
The sad part is that none of these situations (Except Dugtrio, but fuck Dugtrio) would actually kill Volcanion outright or even have the chance of doing so if it wasn't for that Stealth Rock weakness, so in a vanilla setting and the appropriate move, you'd literally win all of those straight up- but you don't, not when Stealth Rock is in play. So, yeah, sure, I guess Volcanion is a "great offensive tank," just make sure you Defog/spin away everything before you use him or else you lose a fourth of your health at minimum.
tl;dr: Great STABs, but can be played around rather easily, coverage options are interesting and fully capable of surprise factor, hazard weakness is a real drag on teambuilding, would probably requires more effort to play well than usual because of it.