CLA angle(s), both sides.
I've never contributed a line to the Linux kernel, but knowing that dead people hold the copyrights for some pieces (nearly) guarantees me that it will never be closed. (It's unlikely that someone could code around those contributions AND get everyone to sign something.)
A CLA could cause a LibreOffice-like split unless it's owned by a stable foundation like FSF (which is different than a company in that it's not owned/bought/sold except in ways that adhere to strict bylaws).
That being said, an early LibreOffice-like split of Mir:
- before usability, means it dies & more Wayland focus,
- after mass-adoption means the split wins (like LibreOffice did).
So I see little concern in contributing as worst-case is either a quick play-out or LibreOffice-style success.