I believe Robin Cook was justified in resigning his Cabinet post under Tony Blair; he was on the right lines. A political party is a complex machine, composed of councillors, activists, MPs, and ordinary members. I hold that everyone who willingly participated in keeping the party machine running under Blair, while aware of his actions, shares some moral culpability.
However, I would exclude Jeremy Corbyn from such culpability; he consistently used his position to speak out in the strongest possible terms against Blair. Incidentally, Corbyn received more general election votes than Starmer. While I am politically a very long way from Corbyn and disagree with most of his social and economic policies, I recognise his basic integrity--something that seems lacking in the post-Blair era.
Labour has yet to genuinely become heirs to government; they are where they are today largely due to the failures of the 'Conservatives', whose name is unearned, as it no longer represents their behaviour, only their rhetoric.