I don’t see river window management protocol being adopted as part of Mir: Mir already has a way to customize window management (see my FOSDEM2026 talk).
For Miriway, I am interested in supporting alternative window management policies and would be interested in how closely RWMP is tied to the river compositor. Isaac didn’t seem to think the current version is ready for cross-compositor use. (PRs welcome!)
And Miracle is already working on a different approach to custom window management.