A good reason why memory virtualization has not been "disrupted" yet seems to be fragmentation. Almost all low level code relies on the fact that process memory is continuous, it can be extended arbitrarily, and that data addresses cannot change (see Rust `Pin` trait). This is an illusion ensured by the MMU (aside from security).

A "software replacement for MMU" would thus need to solve fragmentation of the address space. This is something you would solve using a "heavier" runtime (e.g. every process/object needs to be able to relocate). But this may very well end up being slower than a normal MMU, just without the safety of the MMU.