I disagree.
I never compared Volcarona to Charizard X. I compared it to Y and M-Gyarados. Fact of the matter is that Volcarona, too, can operate even if you fail to clear SR. With bulk, setup, and/or roost, SR is not necessarily an issue if the opponent's team is weakened down or lacks strong enough priority. I don't see why Charizard Y and MGyarados, who also need a lot of support and threats removed to operate on their own, get passes.
Volcarona does provide the utility, or value, rather, that some of the pokes in A tier provide. It doesn't even take up a Mega slot in the process, meaning it has good opportunity cost. We know that Volcarona suffers from 4MSS - many sweepers do.
You're just picking and choosing certain situations in which Volcarona won't be able to run through the opposing team. But, like I said, the same applies to any setup sweeper. The fact remains that Volcarona is a big threat with immense offensive pressure. I'm saying if Pokemon like Charizard Y, which can only hit like a truck and nothing much else (and takes up a mega slot), Mew, and M-Gyarados are considered A, then I don't see why Volcarona isn't when it can shit on every Pokemon A through S save Charizard X, Talonflame, and CB azumaril, whose hits can be decently absorbed by bulky waters.