You know the answer, of course with FPTP there's only two parties with a realistic chance. But why do they? Because you keep voting on them. Your votes made e.g. Corbyn lose but Starmer win. What signal does this give off? A very different signal than if both would've lost. Would another Tory government would have been even worse? In the short term, maybe. But this kind of short-termism is what has got Labour (and all of the other similar parties all over Europe) in this exact predicament. Better to make them lose for picking an awful candidate that's a Tory-lite and bite the bullet. It's not like the Tories would have kept winning for decades on end with the way things were going.