Didn't Jeremy Corbyn run for Parliament in three straight elections under Tony Blair's leadership? Maybe I am projecting the Canadian political system too much onto Britain (though are systems are supposedly closely related)...but in Canada if you were a backbench MP and your leader and government did something you totally disagreed with and if you thought the leader of your party was a "war criminal" - it would be a race to see what would happen first - would you be expelled from the party or would you resign to sit as an independent or to form a new party - there is no way that you could be as critical as Corbyn was of his own leader and be allowed to stay in the party and furthermore if you did stay you would be called a hypocrit for staying.
I can only based this off what I've read from Blair and particularly Livingstone's (who has the same mindset as Corbyn during these years) accounts, but basically the wing of the Labour Party that Senator Blair calls the pacifist Left (Abbott, Corbyn, Galloway, Livingstone, etc.) was still seen as an integral part of broad church Labour and they still all - despite two of them being expelled at various points of their careers - describe themselves as almost tribalistically Labour. Galloway set up Respect but refused to be called to the left of Labour and even had a pop at David Aaronovitch for being an ex-Marxist-Leninist.
Livingstone reckons that the Labour Party was hijacked in the 1990s by what he calls the ''Millbank'' tendency of what we know as the social-liberal Blairites and technocratic Brownites. Livingstone reckons they have always been a minority in the party as a whole, and that a Soft Left candidate would have won the Blair landslides.
Thus, putting this all together, Corbyn probably believes his popularity with the membership, some unions and his leadership position is all he needs to rid the Millbank tendency in the PLP. He has absolutely no interest in leaving Labour because he is more concerned about the interfactional struggle within his own party than even his own short-term electoral success. This is because he thinks whoever controls the Labour Party will eventually lead the country.
