Yes, absolutely we could do that. In fact, there’s nothing stopping a client from doing actor-style parallel execution for performance reasons today, without these changes, for large use cases. Invoking the same contract concurrently gets much more tricky, but yes that analysis is both possible and easier if these changes go in.
As an aside, there’s a lot of optimization that mainstream clients could be doing that they’re not currently. From my cursory reads through several of them, they’re pretty much straight out of the Yellow Paper verbatim, run interpreted, don’t attempt to use natively-sized words, process sequentially, and do gas bookkeeping with at runtime on each opcode call. There’s a lot of room to speed up the existing clients, without needing to wait for eWASM.