The bad thing is that there will be no technology competition. Just HTML and JS for everything? Boring...
This is actually a scary prospect, considering how consistent JS is as a language, let alone as a bytecode replacement.
I expect that to happen sooner rather than later, it is not an impossible problem, and it is far preferable to trying to use JS as a compile target.
I seriously doubt that the world will stagnate at HTML and JS. Thing will evolve!
Though judging by adoption and advancements from W3C, it might be thousands of years ;).