Suppose your party TPK’d in the final fight. Would you reset and try again? Would you accept defeat? Or would you expect your GM to fudge the dice just enough to keep the BAD ENDING from ruining a good campaign? What's your philosophy of failure when it comes to the campaign finale?...
Let's talk about the options you raise first:
Would you reset and try again? If this happens, it destroys the game part of a role playing game. If they get to try over and over until they succeed, then there is no game there - just a story the DM is telling with flourishes from the players. So long as the DM dictates what will happen in major events, the DM is really just playing with themself and is letting the players watch.
Would you accept defeat? You can really build off of this if the players are going to have another campaign following the first. They can experience the world created by the failure of the first party. One of my favorite campaigns, as a player, took place in a setting where my PC sold out the world. We had an artifact. It was essential for the bad guys not to get it. I was playing a PC that was was only in it for themself. The group trusted me with the artifact. A representative of the bad guys came to me and offered me a deal. I took it. The campaign ended that night, and we started a new one soon after that took place 5 years after the apocalypse created when the bad guys got the artifact. It was a fun scenario to explore and we really enjoyed that subsequent campaign ... but it was an anticlimactic end to the first campaign.
To that end, I don't think that a total failure is a perfect answer, either.
Or would you expect your GM to fudge the dice just enough to keep the BAD ENDING from ruining a good campaign?Unless your DM is a master trickster themself, this is almost always obvious and falls into the same problem as the first approach - the DM is dictating the game that is no fun.
So what have I done? You have to respect the way things played out, but you can't let it be a total failure. To that end, I set big end battles up with stages of victory. Even if they do not get the Big Win, the heroes get a chance to ensure lesser victories along the way so that they can celebrate something. As this is the end of a campaign, those lesser victories have to have meaning to the world in general.
In practice, PCs get opportunities to collect resources that make their ultimate adventure easier. If they get those resources, they get a chance to stride in like the hero at the end of the movie that has figures out how to beat the bad guy through more than just force. That doesn't mean that I have not seen campaign ending TPKs ... but they're rare.