2. Are you really sure everything works? (This is a theory that has been tested perhaps twice by one person on one computer.)
3. When/How will proprietary drivers get support? (The FOSS drivers aren't enough for most people with a discrete card.)
4. Is this a plan to gradually convince people to port to Mir? (Mir will have to prove itself to be equal to or better than Wayland, and Canonical will have to do some very creative marketing to win back the devs of other DEs).