Source maps are relatively easy: There's a public standard, a couple of browser makers have agreed to it, and it doesn't need universal acceptance since it's only for development. Basically, it doesn't require a lot of coordination.
A universal bytecode is almost the exact opposite — easy to describe in the abstract, but a coordination nightmare. I've never heard an actionable plan for making it happen. Just getting the community to agree on a standard bytecode would be a Herculean task†, and then you have to get Apple, Google, Mozilla and Microsoft to all say, "Yes, I'm willing to chain my VM to this bytecode at significant engineering expense."
Basically, feel free to work on this if you want, but you can't expect everybody else to hold their breath until it happens.
† You might think that "Herculean task" is an exaggeration, but just look at how much discussion and compromise was needed to get agreement on stabby function syntax for Harmony. And still people kvetched even once there was "agreement"!
Basically, without an implementation, this all sounds like pie-in-the-sky talk to me.
Source-to-source-translation _always_ sucked and that's known for a few decades already.
A designed by committee bytecode would also effectively eliminate browser vendors' ability to compete on speed.
> Source-to-source-translation _always_ sucked and that's known for a few decades already.
>But why is this REALLY COOL?
Because I can potentially debug any language in the browser as long as it has a source map.