While these polls aren't practical -- in that a Biden vs. Cuomo race will not be happening, certainly not in states that have already voted -- I think they are a very interesting measure of the level of dissatisfaction there is with Biden as a nominee. And you can't say it's just Bernie Bros, because Cuomo is ideologically aligned with Biden and pulling better numbers in OH and TX than Bernie ever did.
And yet Biden is doing better in CA, which Bernie won, by more in these polls. There will always be people who wish somebody else was the nominee. Doesn't mean they won't vote for the nominee. And clearly most people are satisfied enough with Biden to prefer him despite the huge boost in popularity Cuomo is getting right now. Hell, I bet if Emerson had gone back in 2008 or 2016 and polled states that had already held their primaries with alternative candidates against the nominee, those alternative candidates would have gotten substantial support as well since those primaries were even more contentious. Doesn't mean anything for the general.