Given what I saw at the MLS working group meeting at IETF last week (as in what'll be all of MLS, not just RFC9420), it's actively being worked on by people who

Given what I saw at the MLS working group meeting at IETF last week (as in what'll be all of MLS, not just RFC9420), it's actively being worked on by people who share your concerns. The higher levels of the stack concerned with "what are all of the options for what's inside an actual message" are under active development, both code and standards work. Those writing code will almost certainly have to change their implementation to be interoperable with the final standards there, but IMO bless them for coming to these meetings with "I tried to implement it, it's broken for these reasons you can see in code here".