Asm.js was never needed as a legacy mechanism, as it was just a compilation target for native code. There was nothing that it needed to remain backwards compatible with, all asm.js code was new code.

https://acko.net/blog/on-asmjs/

OTOH asm.js can be retired now thanks to being backwards compatible with plain JS.

It allowed it to be an experiment that could have been quickly rolled out without a risk of forever lingering as a back-compat requirement for browsers.