The labour right correctly identified the labour left as their primary enemy and purged them. While doing so, they naturally moved to the right on policy. A side-benefit of this move has been to legitimise the party in the eyes of the press, which had made it possible for tory scandals to result in a polling surge. But even if that hadn't happened, the purge itself is the victory. Centrists and right-wingers here recognise this, when they praise 2-time loser Neil Kinnock for giving Thatcher 10 years, but winning the battle that matters.
A vote by the left for Starmer is a vote for irrelevance deeper and grimmer than their current (very dead) status. As the centrists correctly identify, this is a long-term battle. What Reagan and Thatcher show is that this long-term battle isn't won by a series of incremental centre-right, centrist, and centre-left electoral wins, but by a long series of ideological defeats suddenly being reversed by changed economic circumstances. A successful imposition of your agenda means that, for the next few decades, politics are on your terms (FDR, Attlee - Reagan, Thatcher) even if your party is out of power (Eisenhower, Nixon, pre-Thatcher Tories - Blair, Clinton, Obama).
Corbyn had the chance for that once-in-a-generation ideological shift in 2017, but the press and labour right (correctly) tipped the scales by just enough to make him lose. There's no real way of saying when the next chance comes. But Starmer 2024 does not accelerate it.