Your concern with burgeoning standards reinforcing the browser duopoly is well-founded, but unfortunately you're sticking your stake in on the wrong side of the dragon here.
It sounds like you're confused about how Wasm works, and you imagine that it's like a normal assembly language, with calls and returns implemented by the compiler using instructions that push and pop a stack in memory. You're probably right that that's how it should have been designed.
But that is not how Wasm works. The stack is not in Wasm's "linear memory". (This makes it much easier to compile access to local variables efficiently.) Calls, returns, and in a sense exceptions in Wasm are provided by the virtual machine at a fundamental level in a way that makes it impossible for a compiler targeting Wasm to implement tail calls, call/cc, or cooperative multithreading, except by compiling your entire program into a single function, which ruins the performance of existing Wasm implementations.
But, as Scheme has demonstrated, once the virtual machine supports tail calls, a compiler can implement exceptions and cooperative multithreading by way of compiling to continuation-passing style. So this is not the beginning of a long series of extensions; it's Lambda the Ultimate Goto.