The only certainty in this choice between two crappy alternatives is that the consequences of a No are about as difficult to predict as the consequences of a Yes. Maybe a No brings down the government, maybe it leads to renewed efforts for more proportional voting reform. But maybe the Lib Dems, seeing a thrashing at the polls should the government collapse, figure that the only option is to cling on and hope things get better. And maybe the pro-FPTP spin machine (e.g. Tories, most of Labour, all of big business including most media) convinces the overwhelming majority of the public that the people have voted for FPTP. Same thing goes for possible outcomes of a Yes.
It's a shitload more complicated than "No? The end."