If you retract one time, you can retract another time and so on. And in case of Ubuntu, they seem able to retract an impressive frequency.
Ubuntu: "We will utilize wayland"
Ubuntu: "We need to retract: we will utilize Mir because wayland repeats X's mistake"
Ubuntu: "We need to retract again: we will utilize Mir, but wayland does not suffers the same X problems"
Ubuntu: "Mir is protocol agnostic (?) and tailored around unity, so API can be changed every times it needs."
Ubuntu: "We need to retract: API no more begin unstable... bla bla bla"
Who can be so stupid to trust in ubuntu's declaration one more time?