If it had been introduced today, would PNaCl have been rejected which ultimately lead to a better solution (WASM), or would it have been broadly accepted since it had sufficient market reach though Chrome?
Ouch, that's an excellent question, and a perfect example of why the state of web browsers is pretty bleak. Likewise, we could wonder what we will see when it comes to AMP.
Five years on, asm.js/wasm has been completely rewritten, has almost no adoption from developers, and lacks many of the features from PNaCl ... it may turn out well eventually but I don't think it's a great success story for web standards.
PNaCl didn't have adoption either, and the entire foundation was flawed. Starting with LLVM may have made it easy to add features quickly, but the only reason to do that would have been to get early adoption at the expense of long-term engineering benefits. That bet failed.
