Starmer wants rid of the hard lefties - to get into power he needs to capture most the ground that Blair did and this is impossible with the hard left.
He's taking a page from Johnson's playbook - don't back down - he'll chop off the diseased part of the party that is stopping Labour becoming electable or get outed trying.
And if you haven't worked it out yet, he's chopping off the Corbyn luvvies who despite what they say will never change their mind or compromise on their direction or policies.
Labour need to be in power. No matter what they come up with it will be better than what Johnson is doing. But again, the Corbynites would rather remain in opposition, in fight and stand by their principles whilst the country goes to the dogs.