My error was in throwing out some scenario in the hopes of simplifying in order to try and get you to just focus on the question. So I'll reword things and try again.

Full nodes undertake a "UASF" and only accept blocks from the miners that are signaling for segwit and thus bring about a fork.

Now the market has the ability to ultimately make the final decision. Yes, there will be chaos and the outcome would be unknown, but now it's in their hands.

So that means full nodes have zero power?