First, point out a SINGLE sweeper that can be effectively stopped by three completely different types of walls. It doesn't happen. Sweepers are effective for a reason, they are not that easy to stop.Sure, Mega Gengar can have its moveset designed to take out whatever stops your team... as long as that list doesn't include more than 3 Pokémon in all the tiers that have sufficiently varied typings/movepools/abilities/etc. But it's definitely true that having such a variety of things that can run sets to block you would be unusual. The issue isn't having to run all of them on your team to stop Mega Gengar. The issue is that if too many exist, Mega Gengar's role becomes less reliable as there's a chance of its highly-specialised set failing when the thing it's designed to get rid of isn't there, and something else with an equal or at least sufficiently similar effect on your team is. Now you've wasted your Mega slot on an overspecialised Pokémon that can't necessarily do much to the opponent's other stuff.
Except that no one runs offensive Skarmory, because it has horrible stats for it, leading back to the over specialization argument (you're really doing a great job of pointing out why he needs to be banned, so thanks). Even with its best offensive set, offensive Skarmory is a waste of a movepool. And can you think of anything that Skarmory can wall which could also be walled effectively by the set you describe? Especially since it likely won't get setup type when it's thrown in front of the sweeper?What I was saying about substituting a fake in for the real counter to bait Gengar, however, was that you can do something like run an offensive Skarmory while having something else that can wall with screens/Cotton Guard/etc.
What I really wish they'd ban are Hydro Pump and Fire Blast. If hoping an Evasion boost makes a move miss is a luck based strategy, not a true strategy, then the same is true of running a move with <100 Accuracy and hoping it hits.All of that said, you know what? Whatever. While I tend to be against banning much of anything on a conceptual level (especially nonsense like Evasion, where no-miss moves are very commonly available, as are no-miss phasing moves and the new-and-improved no-miss Toxic-used-by-Poison-types, as are things like Foresight, No Guard, Keen Eye, etc.), and as much as I would love to see what would happen if Smogon's entire metagame was rebuilt from the ground up, this thing is almost certain to be a serious problem in the future for many teams. I doubt it'll be much of a problem on PS though, since nobody running Mega Gengar knows what they're doing yet.
But, yeah, Mega Gengar going to get banned. It's very, very clearly unhealthy to the metagame, and your arguments have really been hurting the anti-ban side, not helping it. Arguing "It would completely restructure the metagame and be so cool!" is exactly why it won't be kept around.
You want to know what would happen if the metagame were rebuilt from the ground up? Nothing, really. The metagame did not form by chance. It came from practiced play and observation. It isn't random. Pokémon is a game of almost pure mathematics. Typing, stats, moves, they all boil down to numbers. Pressing reset won't change those numbers. The equations work out the same.