I get that, but narratively speaking, for the audience, it's in the future, so for us...
What's the difference between this and all the other situations in the future where the Doctor has helped out and changed things?
The only difference is the script says it is so, ergo, it is so--that's textbook lazy writing.
And again--WHY are some things "Fixed Points" and "must happen?"
That logic doesn't work, has never worked, and is completely arbitrary.
And granted, this is a show about an alien with two hearts and 13 different incarnations now who travels through time and space in a blue 1960s police box that's bigger on the inside with whatever kind of girl the producers think will make ratings go up and occasionally a guy for the heck of it...all to visit a New New York on New Earth with cat nuns and travel back in time to fight evil trash can tanks with plungers.
I get that you have to suspend disbelief somewhat.
But that doesn't work for EVERYTHING...at some level, you still need your story's logic and universe to have some internal logic stay consistent, at least.
Fixed Points are basically plot points--we, the writers, decide X must happen/not happen this week, ergo, it's a Fixed Point, and even though any other week this problem could be resolved and these people saved, not this week.
That's just weak, lazy storytelling.
Again, in our ACTUAL past, I get--you cannot undo 9/11 or actually kill Hitler...though the Doctor saving his life momentarily is a sight I could've done without, lol...yay, thank you for saving Hitler...then again, for as much as I didn't care for Amy or Rory (the former was useless and grating, the latter useless and dull) at least that gave way to Rory's one good moment, ordering Hitler to get in the closet, lol.
But yeah, I get, even with time travel, you need limits, and obviously a permanent change to our real past is a no-no. But an imaginary future? Tinker with it! You do so EVERY OTHER WEEK! OR, if you really want this to be different and want there to be a reason you can't do this...continuity?
The 4th Doctor didn't kill the Daleks back in time, but hey, that's an imaginary world, you can do what you want, AND it gave a good dilemma, AND there's the in-show reason he even points out, that the Daleks existing will cause people to come together, so undoing them undoes that, so you at least have some justification.
There is none--one episode, they just make up a Fixed Point and a one-off event that must occur because other one-off events must occur later that never matter again. It's lazy, lazy, LAZY.