This always struck me as an odd argument. I hadn't voted for the Democratic candidate since 2008 (all 3rd party), and I hate how they run their primary. But this was EXACTLY what registered democrats voted for in the primary.
There was a primary, and a published, publicly available ticket of Biden/Harris. That ticket had all the deligates, all the funds, all the votes. It was VERY clear who was being voted for and who Biden's successor would be in the highly possible event something should happen to him. When he bowed out and endorsed her, the deligates, acting in good faith... went to her.
Literally any other outcome would have been unfair to the primary voters. Maybe better, but not what they voted for.
I think people would do well to let go of the idea that the parties as a whole have an ideology. George W. Bush and Donald Trump are not remotely the same. Bernie and Manchin are not the same.
Yes there's a good bit of overlap in the venn diagram of their supporters, but MAGA was functionally a hostile takeover of the GOP. The old guard was made to get on board or find a new job.
If Bernie had been elected in 2016 the outcome for the DNC would likely have been just as dramatic. If you want the Democratic party to be a leftist party, go make it one. We've literally seen a major party pushed to fascism. You know know change is possible.