I don't think it's the UI that breaks legacy applications.. it's the rendering engine. Automatically pushing out upgrades to the rendering engine would be as bad as forcing upgrades from IE6->IE8.
I think you misread what I wrote. I was suggesting separate updates for the rendering engine to solve the problem of upgrade-averse users. Legacy applications would be accommodated by keeping older rendering engines running beside a newer, automatically updating engine. The idea is to remove roadblocks to getting everyone to upgrade, and then make it so that upgrading the browser version is no longer necessary to maintain current standards support.