I disagree. The mere presence of Warren in the campaign drastically altered Bernie's style of campaigning and greatly weakened him even before even a single vote was cast. Because Warren and Bernie kept trying to out-progressive one another to win the "progressive/soft left" vote, Bernie did not focus nearly enough attention on the candidate who led the polls for 90% of the primary campaign. Bernie had to spend much of the fall and winter distinguishing himself from Warren. Remember, Warren was ahead of Bernie in the polls for most of the campaign before Iowa. All that time Bernie spent winning back Warren's voters in November/December/January could have been spent on winning over Biden voters had Warren not been in the race. Plus, remember, Bernie's campaign staff still come from the political world/DC bubble. Bernie's staffers saw many of their political-world friends praising Warren or outright defecting to her. Warren's campaign was poor, but it was almost surgically designed to look threatening to the people making strategic and tactical decisions for Bernie.

Secondly, there is a progressive media/organizing/donor class that would have gotten behind Bernie had Warren not been in the race. Look at how many progressive (but not explicitly socialist) representatives and organizing groups either endorsed Warren, endorsed Bernie but only when it was too late, or did not endorse at all. If Warren doesn't run, the Working Families Party endorses Bernie in September 2019 and campaigns hard for him. The Justice Democrats did not endorse Bernie until March 2020. If Warren doesn't run, they jump on the Bernie train in the fall. Would Sunrise Movement had gone for Bernie before January had Warren not run? Tough to say, but possible. Would Ayanna Pressley joined the rest of the Squad and endorsed Bernie (possibly putting him over the top in Massachusetts)? Perhaps. The Nation (the magazine) waited until after Super Tuesday to endorse Bernie because they didn't want to pick a side in the Bernie-Warren fight. Top Medicare for All activist Ady Barkan? Picked Warren over Sanders. Union leader Sara Nelson? Stayed neutral.

Many, if not all, of those organizations and individuals would have backed Bernie from the start had Warren not been in the race.

All of that nonwithstanding, we should also point out that Warren made a concerted effort to harm Bernie's campaign even before Iowa. Sure, Warren didn't have a lot of supporters when she dropped out. But she outright called Bernie a sexist three weeks before Iowa when they were tied in the polls. If you don't think that started turning (white) women voters against Bernie at least a little, you're kidding yourself. Polls in February showed Bernie easily being the second choice for most Warren supporters. But during the final months of the campaign, Warren made up a story calling Bernie a sexist and outright said his campaign is based on a foundation of hate. She said Bernie has zero accomplishments as a Senator. Even groups like Justice Democrats - which stayed away from the Warren/Bernie feud, admonished Warren for her scorched-earth smear campaign against Bernie after dropping out. Warren was calling out Bernie's supporters as bigots even as she declined to condemn someone who invaded a Bernie rally with a swastika. And by the time Warren dropped out, her supporters split between Bernie and Biden fairly evenly (IIRC like 55/45 Bernie).

Warren may be an incompetent campaigner and all around piece of shit, but she did irreperable harm to Bernie's campaign. I would go as far as to say she did as much harm to his campaign as Obama or Jim Clyburn did.