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. I hate using phrases like "their best economic interest" as most take short term gains and sacrifice or risk long term what's best for their economic interests. But in this case there would be some that see it as being in their best economic interest to join in the "UASF".