If it wasn't the security issue laden monstrosity that it is, I could see it living on. But the facts are that the only real "secure" version is the PPAPI impl

If it wasn't the security issue laden monstrosity that it is, I could see it living on. But the facts are that the only real "secure" version is the PPAPI implementation in Chrome (which has it's own limitations), Adobe has dropped future development for embedded systems and that it is almost non-existent on mobile does not speak to the longevity that you predict. Flex is cool, but it's dependency on Flash will be it's noose.