If something is unsustainable it will fail, fall apart, grind to a halt or be overtaken. It is only a matter of time. That's what "unsustainable" means. The problem is defining what constitutes an unsustainable situation, long-term. Something that uses resources at a faster rate than they are produced is clearly unsustainable, but when we're not clear how quickly the resources are produced, it gets tricky. The same goes for, say, a business model that works for now, but not if anyone else starts doing the same thing. Initially, it looks sustainable, but long-term, it isn't.
Mokalus of Borg
PS - Big, complex systems raise this problem all the time.
PPS - And, world-scale, almost all systems are too big to know their sustainability.