// I can't see how Corbyn wouldn't win by default. //
In normal circumstances this would probably be true, but it's far from clear-cut yet! Corbyn has perhaps escaped proper electoral scrutiny for his Brexit position*, and in the meantime the Tories and Labour are basically level in polls. Who knows how an election campaign would shape this? But I think the Tories are currently fairly lucky in their main enemy right now. Unless Leave voters run off in their droves to UKIP, it might be that the Tories can survive an election basically in tact. Or not -- but, weirdly and amazingly, I don't think you can write them off yet no matter how awful the last two years have been!
*I say "Corbyn's Brexit position" in the loosest possible sense, of course.