But you've just said that you understand that Corbyns role isn't PM but as moving the Overton window, presumably for a future leader to win the centre.
This means that you're supporting a leader who you are pretty sure cannot win as it furthers your aim of moving the political conversation.
It also means that you're giving the Tories more years in power. Or to put it another way, you're condemning the poor and desperate to 5 more years of Tory rule in order to move a conversation. How many people will suffer in those 5 years? Is that irrelevant to you?
As ever, we differ on the implementation rather than end goal. If I thought that somebody's role wasn't to be PM then I'd want to remove that person from the leadership position immediately