resource depletion (as alluded to by RWallace) is a strong possible threat. But so is a negative singularity.

Resource depletion is as real and immediate as gravity. You can pick up a pencil and draw a line straight through present trends to a horse-and-cart world (or the smoking, depopulated ruins from a cataclysmic resource war.) The negative singularity, on the other hand, is an entirely hypothetical concept. I do not believe the two are at all comparable.