There could be an issue of opening up even more security issues for people with Flash still installed. That, in turn, will likely lead to an all out campaign to remove Flash from everything possible (maybe not a bad thing at this point).

But, honestly - Flash as a platform hasn't advanced much in quite a while. What it once offered - rich multimedia runtime engine across platforms - is either available in the browser directly or can be attained through even more rich engines such as Unity3D.