That's what I've been saying for years (although only with Tamarin the openness became enough useful for real world application), but there are two problems:

1) Some of the crucial parts for what Flash today is massively used - codecs, some bits of vector processing and shaders (until they switch to pure PixelBender), some of the sandboxing procedures, as well as upcoming HW acceleration are not open, and at least 80% of Flash on the web uses those.
2) If you build your own, better player it will take you a decade to reach Flash Player penetration, so you would need to follow their specifications of what you can and cannot do to the letter. This is the part where disadvantage of proprietary formats kick in...

Either way, it is applaudable for Adobe to release so much of the Flash platform in the open, but it still is not enough (and in most cases it really is not Adobe's fault) for independent groups to release some useful replacement. At present time we still need to look at Adobe for Flash support on our favorite devices.