Yes, but you'd see similar issues (to a different degree) with languages like Typescript and JSX as well.

Cross-compilation has been a thing for a while now -- WASM is the followup to ASM.js, which was already being used as a compile target for languages like C.

Now, reverse engineering ASM.js is easier than reverse engineering WASM (although ASM.js is still a giant pain). And reverse engineering minified Javascript is even easier -- most competent JS engineers could debug a React project without source maps, even if it took them longer.

But it's not clear to me that WASM makes the process meaningfully harder. As in, you're still going to want to use source maps like you use today, and it'll still be totally possible to figure out what a program is doing without the original source. It'll just be a pain.

And the benefits to the web as an open, language-agnostic platform that can be used for memory-intensive tasks outweigh the downsides of needing to work harder to reverse engineer software.

Would you consider the source lost if the (minified) javascript was compiled from TypeScript?

Minifying JavaScript is as destructive as compiling from any language to WASM. The original source are also lost when minifying JS. The fact that the transpilation/compilation target is the same langage doesn't mean its any less destructive.