IANAL but I don't think they would allow anyone to revoke a license after you paid for it, since you could sue them for that.

A while ago, some games were removed from specific markets - for example, you can't buy Bioshock from within Brazil anymore - but before that happened, I bought some of those games.

They could have blocked me from playing them, but they're still available both for downloading and playing - a while ago my HD died and I downloaded all of them again with no problems.