Yeah, me too. That's actually the reason why I generally try to build-in a "finish by" date. Personally, I find that a short but complete campaign is much more satisfying than one that just fizzles and slowly descends into the sad, gaping maw of inconclusiveness.
I prefer to have a game where various plausible endings have been prepared for, but not where one ending has been predetermined. That way, the ending is still likely to get some satisfying conclusions and feel complete, but still open to diversity and surprise.
That's how I run my own game as well. There are certain important things which might or might not happen, certain positive results which could be achieved or failed, certain bits of knowledge which may be acquired or not, certain loopholes that might be closed or left open. All of these things will influence the ending, and most of them remain fully up in the air--to say nothing of all the things that will get added over the course of the game (because about half the unresolved stuff we currently have is stuff I never planned--it's purely arisen from the players' choices.)
Most unfortunate. I have some minor experience with this--a semi-long-runner DW game I played in ended about 2-3 sessions before it would have had its proper conclusion because the other two players were taking care of their nephew and had to get ready for school things in the new school year (because both of them are teachers.)
False normally for me; the only exception was the Ghosts of Saltmarsh game I ran (as in, when we finish the book, the campaign is done), and even then we might swing back to do more on it if the players agree.
A good campaign will have one or more major stories that it tells, and the best time to end one is at the conclusion of such a story. Setting a particular level, number of sessions, or other out of setting reason feels very, very wrong to me. Playing on after the major story is even worse, however, as everything afterwords feels anticlimatic and the game usually just peters out with a whimper.
My first campaign was predetermined to end with Queen of the Demonweb Pits, where the party would face Lolth in her domain. It could have gone differently if the party bucked against the obvious plot hooks, but my group tends not to do that. My most recent campaign was predetermined to end with The Final Enemy, as the primary story was focused on the invasion threat in that adventure. This doesn't mean I don't make changes to the story, but I normally have a goal I work towards.