Which one is everyone going to install: an open source flash player that's not quite compatible, or the adobe one that's perfectly compatible?
Again, I don't see why if there's interest in Flash and Adobe is abandoning its player, the open source ones can't be made to work as well (or as bad) as Adobe's.
Firefox copied IE's quirks back in the day, I don't see why it can't be done again.
What make more difficult the reimplementation is the abscence of a specification or a standard.