People aren't doing that?
Because that is pretty much what I've seen happen.
People take working sites, decide to add a couple of cool features that nobody really cares about, and in the "upgrade" they break older browsers.
Yes, it looks slightly different. But it doesn't look a lot different. Typically the changes that break sites for older browsers are fairly marginal UI changes, that just happen to introduce something that craps out older browsers.
I have come to believe that idealism without discipline is a quick road to disaster, while discipline without idealism is pointless. -- Aaron Ward (my brother)