To be fair, a rock-solid sandbox with extremely well defined and host controlled ingress/egress in a tiny package sounds absolutely fantastic
do not fool yourself with this temporary feature. wasm was first for js performance. secondly for portability. and third and accidentally for sandbox.
It's funny that every time an article on new functionality for eBPF appears in LWN everybody is like "why don't they just use WASM???". More seriously, a forward only language is by definition too strict for the general case of sandboxing, unless you mean using it only for hooking the syscalls rather than as a target for your program. In particular, the Linux implementation caps your execution to 1M instructions, which is not a lot for something you'd want on your browser.