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It seems very clear to me. Sure stuff collapses and gets hotter all the time. We know entropy can decrease in isolated systems, so long as the total entropy of the universe remains constant or increases. This isn't a problem. But if EVRYTHING is collapsing, this is a very different scenario. Entropy is decreasing. If we assume the universe was a singularity before the big bang, this singularity is literally the lowest entropy possible in the universe. If this is the ultimate fate of the universe, then the total entropy of the universe must decrease to get there.
If we can say with 100% certainty that entropy cannot decrease, then that's the same as saying that the universe cannot collapse into a singularity. At least, that's how I interpret it.
Thinking more about this. An expansion-contraction cycle implies increasing entropy then decreasing entropy... why seems to violate the laws of thermodynamics. I'm going into speculation mode here but my best guess would be that the total entropy of the universe is constant, and we just happen to occupy a region where it increases due to expansion. In other regions of the universe, contraction is happening which perfectly balances out the net entropy. This idea implies the big bang is still happening... simultaneously the big crunch is happening elsewhere. The torus model of the universe is a potential model that allows for such dynamics.