I think youre referring to PNaCl(as opposed to Native Client), which did away with the arch-specific assembly, and I think they shipped the code as LLVM IR. These are 2 completely separate things, I am referring to the former.

I don't see an issue with shipping uArch specific assembly, nowadays you only have 2 really in heavy use today, and I think managing that level of complexity is tenable, considering the monster the current Wasm implementation became, which is still lacking in key ways.

As for out of process sandboxing, I think for a lot of things it's fine - if you want to run a full-fat desktop-app or game, you can cram it into an iframe, and the tab(renderer) process is isolated, so Chrome's approach was quite tenable from an IRL perspective.

But if seamless interaction with Web APIs is needed, that could be achieved as well, and I think quite similarly to how Wasm does it - you designate a 'slab' of native memory and make sure no pointer access goes outside by using base-relative addressing and masking the addresses.

For access to outside APIs, you permit jumps to validated entry points which can point to browser APIs. I also don't see why you couldn't interleave stack frames, by making a few safety and sanity checks, like making sure the asm code never accesses anything outside the current stack frame.

Personally I thought that WebAssembly was what it's name suggested - an architecture independent assembly language, that was heavily optimized, and only the register allocation passes and the machine instruction translation was missing - which is at the end of the compiler pipeline, and can be done fairly fast, compared to a whole compile.

But it seems to me Wasm engines are more like LLVM, an entire compiler consuming IR, and doing fancy optimization for it - if we view it in this context, I think sticking to raw assembly would've been preferable.