Even discounting the changes in the ex-industrial former "red-wall" seats, no Labour leader has ever become Prime Minister without winning at least half the seats in Scotland. Even if Scotland does not vote for independence in the next few years, Labour is never going to recover there. If independence comes then it is academic anyway.
Combine that with the changes in the former Labour seats in the north (which I think will only continue) then a Government cannot be formed of London MPs, a few larger cities and some seats in university towns/cities.
I don't think Starmer is the most charismatic leader but I think any ppolitican would struggle to solve this. Very hard to try and appeal to ex-industrial seats AND young, metropolitan, middle class voters in London etc.
One of the best comments I saw yesterday was Labour trying to make traction of the conservatives being corrupt and stuffing money into places that voted for them, was people thinking "well all politicians are corrupt anyway, so we may as well vote for Boris to get some of that too!"