I am actually buying another licence right now, since I love developing with it and want to use the framework on one of my bigger projects. I'm worried there is an outside chance I might not be able to after the 5th (although it sounds unlikely)...
From what I've heard on these boards (albeit an often biased opinion), the worst situation is that you are left with a versions of XenForo that is then further developed by someone else (if at all), not that you will have to take down your forum
So I'm buying another licence based on that (and that I might not be given the chance to buy it after the 5th at all)