Why having everything at stake means nothing is at stake
You know how the series finales have been getting more and more high stakes? Like, first it's the Daleks coming back and the Doctor maybe killing most of the people on Earth. Then it's the Daleks and the Cybermen taking over and killing loads and loads of people. Then it's the Master turning Earth into a base from which to take over the universe. Then it's Davros destroying the whole universe. Then it's the whole universe being destroyed and never having existed. Then all of space and time collapsing in on itself.
These consequences all have one large thing in common: they could never happen. The BBC would never have their number one program kill off most of the Earth's population and just continue on like that. And the show couldn't continue at all if the universe never existed. So there's no question that everything will be fine in the end. And now it's more than that. It's not just that everything is fine in the end, it's that nothing was wrong in the first place. The giant reset button has been getting a lot of use since the Year That Never Was. You can only get so much excitement out of a situation you know can't go wrong. You can usually be decently sure the good guy will win, but you don't always know at what cost.
Recently, the cost has been nothing. No losses, no consequences. Everyone's fine, everyone's alive, home in time for tea. Frankly, it's boring. And it's not how life works for anyone. Friends die, opportunities are missed, mistakes are made that can't be fixed. The point of science fiction and fantasy isn't to make up a world where nothing ever really goes wrong, it's to play out realistic problems in a fantastical world. Bad dreams that you wake up from and realize were never a threat don't count as realistic problems, but that's the amount of tension Doctor Who has largely dwindled down to. There have to be defeats for victories to matter.
