Wow, there are a lot of people reading this who don't know squat about software development based on the votes.
When you throw away old cold, you throw away years and years of bug-fixes. If you genuinely start from scratch you have to find and fix all these things again.
Lots and lots of companies have made this mistake, and many of those never recovered... they lost years working on it only to have something that was not as good as the previous iteration despite looking shiny.
Biting the hand that feeds IT
