Is MLS required by law/EU or something like that? I wonder if this will be the type of thing that will only be implemented by "mandatory" phone messaging apps, and never adopted by the big players like Whatsapp and Messenger.
Several Facebook people (@fb.com) were taking part in protocol design, so I wouldn't say they'll never adopt it. It all depends on how much shit the EU is willing to take when it comes to interoperability.
MLS without MIMI isn't going to change much, though, and MIMI is still a work in progress.
(More Instant Messaging Interoperability)
Indeed MLS alone is nowhere near good enough to make systems interoperate. Here's to hoping MIMI takes significantly less of a fraction of a decade to get built that MLS took (5 years since chartered, 3 years after initial due dates).