Well, for one thing, mature debuggers exist (the equivalent to the tooling he inquired about for .wasm).
More importantly, however, anything GPL must make source available and reasonably accessible. There is no such guarantee or even expectation for random programs on the web.
Indeed, huge amounts of JavaScript that’s already out there does not come with a free software license. The FSF has been complaining about this for years.
As for debugging, this is not a particularly hard or fundamental problem. It’s basically solved already.
