I would say Labour under Blair was centre left. Currently it's not even that, it's centre-right.
But regardless of labels, if you don't see the socialist, corbynite or Your Party left as belonging as a valid part of the Labour party, contributing to the party's voice and policies, then surely you can't accuse it of "splitting" the vote and being responsible when the Labour party doesn't get elected. Because it's not a single, even potentially unified vote bloc to be split, according to you.
That would just be saying "there's this certain area from say 20% left to just right of the middle that belongs to the Labour party, and votes together to try and form a Labour government. Those on the right of it are represented by the Tories and Reform, and those on the left of it have no right to representation whatsoever!
That's not supposed to be how democracy works. Labour, like any other party, can make its decisions about how broad a church it will be - who it accomodates into its political compromise, and who it considers beyond the pale and exludes from it. Once it's decided a certain set of people are excluded (as you seem to believe the left should be) then it/you can't complain about them seeking representation by other means. Particularly when they were perfectly happy to rub along with others and its the others who decided against it.