Well you don't need to implement every stupid thing big G thinks should be in it, just the really critical stuff. Even if you freeze all features right now you'll probably still have a better renderer than gecko for 5 years into the future.

I mean right now I bet a lot of people will simply not update to MV3 and continue using the last known MV2 build into perpetuity until certs break or something else. I sure intend to.