In my experience, players who are enjoying a campaign will happily keep going as long as the DM wants to keep running. It's the DM who burns out and wants a change. Not saying that is universally the case, but that's how it has always been in my groups.Dausuul I want to ask, have the players ever wanted to continue playing after the idea ran it’s course? It reads like it has from your post.
(Of course, if the DM has burned out and exhausted the original campaign idea, the quality of the campaign is apt to decline and players may stop enjoying the campaign as a result. But it takes a little while for this issue to become apparent.)
This may or may not be connected to the fact that the groups I play in always have multiple DMs. I know there are groups where one person is the permanent DM, year in and year out; and maybe burnout works differently for those folks. I couldn't do it. After 12-18 months behind the screen, I want to step back and sling some dice and kill some monsters.