Well...yeah, but if Labour lose again then that's another half-decade they've got to wait until they get another shot at winning an election. When going
into an election they need to be doing all they can to maximise their chances of actually winning. Going into one with a leader who can't mount consistent polls leads and who is deeply, deeply unpopular is literally doing the exact opposite of that.
For what it's worth, I do think there's an annoying tendency for people to cling onto every single poll that comes out as representative of whatever grand narrative they themselves support. And there should be a recognition that even electable leaders will go through phases where they aren't well-liked for whatever reason. But that's different to when an opposition leader consistently fails to mount substantial polling leads, which is
generally, like, a basic prerequisite for any opposition party who wants to come into power. And similarly, any opposition leader coming into power is generally going to want to have fairly strong popularity ratings. Corbyn...doesn't. And to point that out isn't being overly-dependent on polling, it's just stating basic facts which highlight he isn't doing well and that his current approach isn't working. If you're heading into an election, you generally want to do all you can to minimise your chances of defeat.
He did well in 2017 to claw back from almost certain defeat, but that was then. There's no guarantee he'd be able to do so again. I reckon Brexit's soured a lot of people on him to an extent that'll be hard to recover. He's
generally a strong campaigner but there's a possibility he's largely maxed out his potential voting base and that he's not been able to successfully expand it at all. And for as much as they remain in disarray the Tories will struggle to run a campaign as bad as the last one. And some of Corbyn's lines of attack may struggle to be successful because he's literally been supporting the process which they've cocked up more than anything else in the last couple of years.
And on that last point - I wouldn't even class myself as a nationalist!
Just generally prefer the SNP's base level of competence which Scottish Labour have typically lacked, and believe we're too better off as an independent country. It's sort of hard to explain if you're not in Scotland how shite Labour have been - and continue to be - up here.