> Nobody wants another fat runtime that is as entangled with the browser engine as the JS engine is.
Why would it need to be? There's no reason to assume the architectural decisions for javascript have to be repeated for WASM.
The runtimes themselves could be WASM binaries cached like any other resource, or managed like packages. Javascript could be disentangled from the browser by doing the same - just treating it as a resource that ships by default with the browser.
I don't know if that's feasible - maybe not - but I bet there are possibilities other than "ship the entire runtime with each script" that can be considered.