Full nodes undertake a "UASF" and only accept blocks from the miners that are signaling for segwit and thus bring about a fork.
Well, then these nodes will stop if the block chain has a single block on it not signalling segwit.
That's based on the assumption that at least some of the miners that are currently signaling for segwit would not "switch" (at some point at least) to build the "segwit" blockchain.
That block will never come.
Again, you're basing that on an assumption. It will never come if all miners stop mining their current block when they receive the next one, i.e. they accept the next one no matter what. A miner that joins in the UASF will reject the next one if it's not signaling for segwit and continue mining his block.. So as far as he's concerned, he starts producing 100% segwit blocks. His chain never contains blocks from those that aren't signaling for segwit. And since he's broadcasting his blocks to the the nodes, those full nodes will take them and continue on. So of course it will come.