I've decided that without the tenth Doctor's TARDIS there to crash into, the
Titanic crashed into the fifth Doctor's TARDIS instead-- we know they were in the same area thanks to "Time Crash." So in the "Turn Left" universe, the fifth Doctor does his best, but it's (as usual) not good enough, and as the
Titanic plunges onto the Earth, and he can't do anything to stop it, he jumps into his TARDIS, murmuring "There should have been another way..."