I may be over generalizing but this is so true. The same People will yearn for the existing UI when you Google updates it six months down the line.
may be the existing UI works fine, and having invested the effort in using it only to be forced to abandon it once again.
Imagine if a car manufacturer keeps changing the wheel and the pedals around.
Why don't they change the UI, then? Mostly because of regulations, I think. It's unclear to me whether bringing a new driving UI to the market would be horribly difficult, costly and risky, or just plain impossible.
While you and a million others might not like the new $webapp UI, it's still a good thing that they can and do get changed. Especially in this case considering you can start using any email client anytime if you don't like the web UI.