I'm inclined to disagree. I know what you're getting at, but I don't think it's anywhere close to "one of the worst arguments ever." We have two tools with which to make this decision-- our theorymonned knowledge and our actual experiences, so let's not entirely undercut the value of the latter...
There are things that sound absolutely brilliant in theory, but aren't flawless in practice. Wobbuffet was theory-banned during the shoddy battle days even though it seemed like generation 4's offensive threats had caught up to it; there was no statistical data to back up the argument (it wasn't being heavily used) and it was because of a vocal minority that repeatedly painted scenarios of wobbuffet's "unbeatable nature" that it ended up getting the axe.
But really, you had to play optimally to get those kinds of results. You also had to correctly anticipate the moves of your opponent to achieve the best results. The nature of the tools you were using allowed forgiveness from play errors, but at the end of things, player skill was maximizing your results.
I feel like Mega Gengar falls into a similar --although obviously not the same-- kind of camp. Nobody is questioning that Mega Gengar is a very strong tool. But it's a tool that rewards good play on your part and punishes the mistakes your opponent makes-- Yes, you will get into situations when piloting it that you are guaranteed kills if your opponent plays into your hands-- but those guarantees are still play-skill dependent.
You need to account for Mega Gengar in teambuilding. I know others aren't into it, but I'm a huge fan of sticky web, which allows positive-natured, fully invested base 78s to outspeed and get the jump on Mega Gengar and smash its face in, should it switch in grounded. I'm also a huge fan of the mixed, pivot aegislash, who is capable of revenge killing with shadow sneak, but-- more likely-- will punish the opponent's "free" kill, and uses gengar's switch-outs to launch impressively-powered shadow balls and sacred swords at incoming switch ins.
Interplay between megas is something I feel shouldn't be discounted when deciding the fate of Gengarite. As others have mentioned, in choosing gengarite, your are passing up the opportunity to use mega kangaskhan, as well as any other megas which could become topically good. That's a very real cost-- I feel that when we start banning mega stones, we'll be running through quite a few of them because we'll be removing this cost (i.e. "now there's no reason not to use mega kangaskhan, so we have no choice but to ban that"). Since mega stones are a new mechanic, I feel they deserve a bit more time under the microscope, because I'm not convinced we have appropriate respect for some of the teambuilding decisions they require.
It feels true that Mega Gengar is best billed as a team player, who opens the doors for threats to take advantage of the holes it creates. But to do that, you do have to take some teambuilding into account, however minor... Mega Gengar best helps a number of key offensive mons. When you start seeing the same few together, it really drives home the impression that this is a team archetype we're talking about-- that traps and dismantles opponents-- rather than a Blakizen-esque mon that runs the show by itself. We made great strides in generation V to balance what we felt was a major characteristic of competitive play-- infinite weather-- so considering the threat this quickban would have towards other mega pokemon (and maybe you'll laugh, but also the removal of the offensive trap team archetype, if I'm not dumb and that's really a thing) makes me believe that a quickban would be a hasty decision here.
Getting your kill-value up-front is really strong, no question, but other Megas like Kanga deliver value that is greater and while it's not a sure thing, it'd argue it's all-but-guaranteed unless the opponent is packing specific answers.
I think Mega Gengar is insane, massively powerful, and extremely hard to deal with in the right hands. That said, I'm not sure it warrants a quickban.
Thank you for reading.