The new leader has a poisoned chalice admirably outlined by an opinion piece in Unherd.
https://unherd.com/2020/04/the-last-thing-labour-needs-is-unity/


Unity. According to Sir Keir Starmer, it’s the pre-requisite for Labour to re-establish itself as a serious party of government. As he put it recently (and has said repeatedly): “I don’t think there’s any victory without unity.”
Instead of seeking to assuage the anger of Corbynite members because their hero will no longer be leader, Sir Keir should confront them and — as Neil Kinnock did with Militant in the 1980s — highlight the grotesque chaos of having such people in a mainstream party.

Had Neil Kinnock not fought and defeated Militant, Labour would have ceased to exist as a mainstream party in the 1980s. The situation today is of course different, but the same dynamic holds; if Sir Keir refuses to fight the hard Left, Labour will continue on its current trajectory and cease to exist.
It’s obvious why he won’t. For one thing, the hard Left now dominates Labour’s National Executive Committee by 21 votes to 13