It's like when they replaced the first two M's, about whom there was some debate whether or not they were intended to be the same person, with Judi Dench, creating a clean break with the old. You can't just continue pretending a character like that is the same person when the actor changes, especially when the Bond actor changes every 5-7 films. Besides, no one could ever replace Desmond. Best to bring in a new character with the title of Q.