Its an obstacle for the plot to only overcome when the writer decides it can be so. Its just another plot mechanism.
Yeah, much like how the "deadlock seal" is an arbitrary weakness to stymie the sonic screwdriver for plot's sake, "fixed points" help explain why someone with a time machine is beholden to existing historical events.
I expect points get more fixed the closer you get to them. So it's easy to change Waters of Mars from Elizabethan England if the change is large enough (blow up the planet). But once you get right up next to them you don't have enough "leverage".
I like this explanation. It's a bit arbitrary but no more so than anything else in this conversation. And there's a certain elegance to it.
The new series has come up with several ways to fill in the plot holes that would otherwise be caused by the simple fact that
they have a time machine! One of Eccleston's early episodes had Rose ask why they couldn't just use the TARDIS to go back to before the catastrophe started and he just said, "We can't. We're part of events now." And then there was the exchange between Amy & Smith in "The Angels Take Manhattan": "Time can be rewritten." "Not if you've already read it." I like both of those explanations better than the "fixed point" stuff.
While the classic series never spoke about "fixed points" per se, Hartnell's Doctor did seem particularly perturbed whenever someone tried to change history, whether it was Barbara's attempts to reform Aztec society in "The Aztecs" or the Meddling Monk attempting to turn away the Viking invasion in "The Time Meddler." But there were other times when he seemed to get caught in predestination paradoxes, like when he inspired Nero to burn down Rome in "The Romans" or when he & the Daleks inadvertently caused the crew & passengers of the Mary Celeste to abandon ship in "The Chase."
As for "Waters of Mars," I wonder if Adelaide Brooke would have killed herself if the Doctor hadn't kept mentioning the fixed point stuff to her. First he acts so fatalistic and uses the fixed point to explain why he's going to let them all die. Then, when he finally does save them, he screams like a madman about how time will obey him. That's bound to confuse anyone into making rash decisions.
But I also thought that Adelaide Brooke's suicide never made much sense. Originally, she was an inspiration to her granddaughter because of the mystery and romanticism of dying in space. But when she kills herself, how is that supposed to inspire anyone? If anything, I'd think that it would make her granddaughter go, "Space? Oh, you mean that thing that made my grandma go crazy and kill herself? No thanks. I'll pass." I mean, I expected there to be some kind of UNIT cover-up at the end of the episode to conceal the Doctor's involvement in the whole thing. Like, they would just pretend that Adelaide had died on Mars and keep the surviving astronauts in seclusion for a few months, then pretend that they had used the rocket to return to Earth.
