So, the second time it happened, after nearly two years of campaigning, we realised that the game was starting to become stale but were reluctant to let it finish with a whimper. I asked the players 'if you knew the game was going to end over the next two sessions, in what manner would you like it to end?'
After feedback, I then designed a fitting, epic ending that tied up the very few plot threads that I'd allowed to grow. We all agreed that, even though it may not have felt like a full and proper campaign, we had at least given it closure and weren't always having 'maybe if we'd done this...' conversations.
I feel that properly finishing a campaign is the right thing, but if you have to cap an ending on just to finish up a failing campaign then do so, or that sense of incompleteness can leave a nasty taste and even have a negative effect on your attitude towards the next campaign.