Let me answer this as someone who writes non-brittle Javascript code on a 1 year old web app (e.g. I have more unit tests per LOC in my Javascript than most of

Let me answer this as someone who writes non-brittle Javascript code on a 1 year old web app (e.g. I have more unit tests per LOC in my Javascript than most of the Java code that's in the underlying app. AND I run my test suite on all major browsers regularly). Even if you write solid code, using good architecture, and thorough testing you will find that if it passes on FF then there's a 98% chance it also passes on Chrome and Opera... but maybe a 60% chance that it passes on IE8 (requiring HOURS of work to investigate and fix), and there's some abysmally low rate of success on IE9 from having run these tests against the Beta earlier (we don't plan to announce IE9 support for a while). So while you are correct old cruft will have to go, brand new solid code shouldn't have to go as well... and that's unfortunately the consequence of IE9. :'-(