Ordinal M.

I did like the recent Who episode ( The Girl Who Waited ) but it also pointed out to me quite how far Who has gone in terms of absolute nonsense about time paradoxes and, particularly, "if we do X, Y will never have happened!" - something which was being subtly attacked in the plot. Whenever you hear this phrase in a Who episode you can be sure that it Does Not Mean What You Think It Means.

( Obviously, spoilers, but say that in a River Song way and I will remove your face )

Now, time-travel-wise, what is really being happening is moving between different timelines; in that case, yes, you can have people who remember things from one whereas everyone in the new timeline doesn't, you're moving between similar universes. But you know what happens if you "collapse a timeline"? You destroy everyone in it. They did exist, and now they don't. The "universe reset button" in the last series basically killed everyone apart from our three protagonists.

There is an alternative to this, of course. Perhaps these paradoxical events really did "never happen". What we are doing in watching Who, in this case, is following the delusions of a series of people who are having acausal experiences not connected to external reality. Everything they remember having happened never actually happened and was all in their heads. Perhaps they are brains in jars, too. (This level of philosophy of mind also makes bad drama, as well, I admit.)