You are posting your opinions as facts in this thread and it's quite annoying to read.
I am doing no such thing. I have absolutely no investment in Chansey's ban one way or another, and I recognize that the individuals who do not want it banned have valid points.
It's just that the vast majority of people posting here have not been making any.
His argument isn't asinine at all. Chansey can't do anything on its own except maybe hit something with status, and if you are a very good predictor it can pass Wishes sometimes too. In turn you get a pokemon that is set up on by most of the tier and makes your team easy to predict around. No competent team is going to look at the team reveal screen and say "shit, they have Chansey, now I can't win".
There is quite a lot that's wrong here. First, since when is spreading status, setting up a hazard, or passing wish/using aromatherapy considered "can't do anything on its own"? Because every time Chansey comes in, it will probably be doing one of those things.
Second, how is Chansey set up on by most of the tier? In order to set up on Chansey, you either need 101 Substitutes (or a ghost typing with a sub), Taunt, the ability to ohko Chansey, or some way of not caring about status (i.e. Rest). These are not qualities the majority of Pokemon in the tier typically posses. And even if we do assume that Pokemon that can deal with Chansey are running amok, so what? Chansey's entire job is to come in on the "significant portion of the metagame" it irrevocably walls and support the team, and it does that regardless of whether it has a counter waiting in the wings. This is why I keep repeating ad nauseum that we cannot apply the same logic towards defensive suspects that we do to offensive ones.
The only way a Pokemon can truly be said to counter Chansey is if its switch-in steals significantly more momentum than whatever Chansey just did, and there are a couple of these around; Mew was mentioned, as was SubKyurem. These are Pokemon so dangerous that the free turn Chansey affords them is a lot worse than any status it may have spread/healed or wishes it may have passed. But they are not common, and they are definitely not "most of the tier".
As aeroblacktyl/mop alluded to, taking out Chansey leaves most teams that use it with a gaping hole because they use it as an all-purpose special wall. The defensive characteristic of banning something implies that a pokemon can wall things under common conditions. Chansey can't do that, it is dependent on its teammates probably more than they are on it.
You just took a flying leap over a metaphorical chasm of logic, my friend. How does the rampant exploitation of Chansey's ability to wall the vast majority of the metagame allude to its inability to wall things under common conditions? It's true that Chansey is often a team's sole stop to a large number of very dangerous sweepers, but I feel like that supports my argument far more than it supports yours...
Your point is that pokemon is a team game and therefore every broken pokemon needs team support, and I think it doesn't make sense. Something like Lugia or Giratina (some of the few broken defensive mons) doesn't care about what else is on its team, they are going to wall almost everything no matter what.
I can guarantee that an otherwise poorly made team with one of these "broken walls" would do very poorly against competent players in UU. As I outlined in an earlier post, the standard Giratina-A is actually defeated by quite a few pokemon in UU, including the ubiquitous Kyurem, the amazing stallbreaker Mew, Houndoom, and more. No, team support is far more important than you're giving it credit for. I would even argue that it is built right into the "common battle conditions" aspect.