In practice, would Google ever dare invoke this on a large scale? No, you know that. Yes, there should be some transparent process that is undertaken if yours were revoked, but Google only like stuff that scales, manual processes like that don't.
And really, if the ability to revoke your URL at any time wasn't in the T&Cs, do you really think it would make any difference to the revoke rate?
Sure, nobody in their right mind would depend on such a URL sticking around, but this post isn't "Why I won't get a Google+ Custom URL", it's "Why I won't use a Google+ Custom URL as the primary mechanism to link to myself online". I don't see much argument against just claiming one, if you're already a g+ user.