> You can compile several languages into WebAssembly already. Languages that can't be compiled will eventually have their own interpreters compiled to WebAssembly.

A big constraint here is the memory model. Languages include a specification for memory allocation, deallocation, lifetime and garbage collection, and WASM has it's own way of going about that that is engine dependent. The performance lost from reimplementing the memory model within WASM could only be regained by going back to something that looks like containers.