Of course it's complete guesswork - as is your assertion that there would have been no impact on their vote. I suppose it's fair to say that, given the outcome that we've observed, they'd have been better off with the uncertainty of the alternative, but we don't honestly know what the response...
If they had a genuine shot at some power, and refused on a "point of principle" which I'd imagine a decent amount of their supporters didn't give a hoot about (PR), what do you think would have happened to their vote at this GE? It would have been clear that they were a protest party, without...