I think Starmer will win. The legions of young people who joined Labour under Corbyn also tend to be passionately pro-EU and i think he would have won their hearts by being such a great Labour shadow Brexit minister
I think it could go either way. He is not quite as left wing as Rebecca Long Bailey and more articulate. But his disadvantage is he is from London which is swinging towards Labour and much of the focus is on winning back the Midlands and Northern areas they lost. At the same time Brexit will be a non-issue by next election so whether those swing back or not will depend on how Johnson governs and Labour's platform. I think the suburban and urban ones they had a good shot at winning back, but the more rural ones like Basseltaw or Bishop Auckland, I think those might be gone for good or at least only in a landslide will they flip back and I don't see that happening next time. If they lose next time around, then maybe in 2029 or 2028 Tories will have been in power long enough for a big shift to happen.