In my opinion, throwing away five years of development, when you have a working product, is a terrible idea.

So Microsoft shouldn't throw away their IE code? ;-)

Also, you wouldn't have to throw away the other code at all, if someone's interested in starting a new project, go for it. If the code ever reaches the point that it's superior then use it. You don't have to decide to switch it before the alternative has arrived. Did you hear people saying "I'm going to switch to Linux" after Linus' first email on the subject?