It’s not about debugging, it’s about the legal "right to decompile", which is intended to give customers and companies the right to take apart products and software they bought (even from competitors), and gain knowledge from that.
It’s also intended to allow them to modify it for their own purposes, and provide the modifications to others who have a license to the original product.
Especially on the web this was finally possible, and now you’re suggesting to take this away again?
You don't seem to want a right to decompile/disassemble (which you could do in the same form for WASM than for most other languages) but a right to directly view the original source code. Such a right does not exist in general. The owners of the source code (the intellectual property) may give you the possibility to view it, but they could also decide that they don't give it to you. The only exceptions I know of are if the creator incorporates other software parts which mandate releasing the source code, e.g. GPL libraries.
Many countries allow reverse engineering, and you could infer a "right to decompile" from that in some cases, but as far as I know none of them place any requirement on the source to make it easy.
At most they legally protect someone who decides to run a disassembler or decompiler on a binary from prosecution or lawsuits (but only in as much as you don't use the result to violate copyright or other restrictions).
What's a "valid" source map anyways?
Furthermore, debugging with sourcemaps is suboptimal. WebAssembly wants to support much better debug information than this.