One of the big issues for a WebAssembly-First programming language becoming big is that Rust is already has a very good WebAssembly story, being a direct compiler supported target.

In addition, WebAssembly was designed to be able to run languages like C and C++ (and Rust) efficiently, and so does not have built in garbage collection.

Thus a language that wants to be WebAssembly first, is either going to have to bring in a garbage collector, have safety with something very similar to Rust's borrow checker (in that case, it would be similar complexity to Rust), or else put the burden on the programmer for safety like C and C++ (in that case, what is the advantage over them).