When Brexshit goes tits up, as it may well do by 2024, that brush may not be quite so well tarred by then.
I don't think Labour will have too much difficulty winning back seats from the Tories that they lost in December with any candidate beside Long-Bailey.
Both are quite wild assumptions - that Brexit will have such a negative effect on the economy to swing ex-Labour seats where there's very little youth vote around and where people are mad at Labour more than anything else.
The likes of Crewe & Nantwich, Dudley North, Great Grimsby, Newcastle-under-Lyme and Scunthorpe for example don't look like assailable majorities to me. Let alone those the Tories flipped in 2017 against the national swing (Stoke South, Middlesbrough South, Walsall North and Mansfield).
If Labour start going after those seats while ignoring the seats they narrowly lost in 2019 that they didn't previously hold or narrowly held. Going all-out to win back "Bolsover coz it's Labour" for emotional reasons is not what Labour need. Preventing a majority is going to be done by winning back the Birmingham seat they lost along with Burnley, Bury, Peterborough, Stoke Central, West Bromwich, Wolverhampton et cetera. Lots of suburban London seats are also in that category. Way more fertile ground for the party.