Backwards compatibility is always an important point when it comes to companies, because
the upgrade cycles are normally rather slow (due to security policies and the good ol' "never
touch a running system" piece of wisdom, which especially holds true in a productive
environment).
Plus there's a lot of people out there who would probably not be able to tell you what
version of the Flash Player they're using, much less know they can / should upgrade, or in
the worst case, even know they have it installed (or even know what "installed" means...)
Keeping it running on two tracks, one for FP9 and one for FP10, would be a lot of pain, as I'm
planning to make good use of the new features FP10 offers, meaning (pretty much) all
Arrays will be replaced by Vector.<>s, and I'd spend some time looking into the TextEngine
to see if proper support for right to left languages could be implemented, and maybe even
things like font embedding etc.