I've been in this situation a couple of times as a GM: the first time, after more than a year in the campaign, we all realised that the game was losing it's shine and we weren't as committed as we once were, and we let the game fizzle out. Although it was a relief to be able to let it go - too many plot threads and avenues of possibility - there was, much later on, a twinge of regret. Regret at not finishing the campaign, at not resolving certain character arcs, and not giving a proper ending to a game we'd been playing pretty much weekly for over a year.
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.