I'll preface this by saying I'm not a huge believer in "Sanders staying in hurts the Democratic Party" line of thought. As long as he's campaigning on the issues, which he has this entire time, he's just shifting the Overton window. Biden will have a chance to keep looking like the moderate and reinforce his electability argument by continuing to rack up wins.
However, I am starting to think that making this a quixotic run ("we can still win this") instead of a principled one ("we want to keep progressive ideas on the ballot and help downballot progressive candidates") hurts Bernie and the progressive movement in general.
Bernie needs almost 60% of the delegates (991 of 1677) just to get a plurality. He has not hit that number in any state thus far, including his home state of Vermont. The list of states remaining isn't favorable for him, either, going for Hillary in 2016 by an aggregate of 165 delegates (15 HRC wins, 10 Sanders, 2 ties).
To summarize, he needs margins larger than he's gotten anywhere in 2020 in a slate of states that didn't support him previously while polling 20 points behind. As a result, when he says "there's a narrow path", it doesn't sound realistic.
"Not sounding realistic" has always been the biggest line of attack against progressive candidates in general: sure, it'd be nice to help people, but it's an unrealistic fantasy due to (practical/financial/political) reasons. If your biggest weakness is that people think your positions aren't realistic, don't say other things that sound unrealistic. It just reinforces people's opinions of you and, by extension, your positions.
If you want to stay on the ballot so your voters show up for downballot progressives, that's great! Just say that! We need more progressives downballot. We need the next generation of progressive politicians at the state level who will become the future representatives and Senators and presidential candidates who will get the progressive agenda enacted. If you get asked about whether you can win, say every voter deserves to have their voice heard. Nice, simple, principled.