This kind of logic is weird to me, given that the reasons people think Biden will likely lose to Trump aren't baked into the polls. People like Biden because of nostalgia for Obama but once they understand who he really is after months of news stories and ads hammering on his record and bizarre behavior/gaffes, it's going to be a much tighter race at best.
That said, I was much more confident in "Biden loses to Trump" before the COVID-19 situation, which may change everything.
I'm gonna be real with you, I just don't find "but Biden will have to deal with months of negative campaigning!" that convincing as it ignores that literally every candidate in existence would face that same kind of scrutiny. Bernie would have his past statements and votes brought up too.
And while Biden's said and done some rancid shit, it all gets trumped by "he was Obama's VP for eight years," full stop. Anything he did before that might as well not even matter. I could very well be wrong, I just don't see any attack that draws on pre-2009 Biden's record being very effective.
Anyone watching the polling numbers in 2016 would have known Hillary had some massive liabilities going forward, and that just hasn't been the case for Biden. Bernie's -14 favorability and Biden's +3 mirrors Bernie's +3 and Hillary's -14 from this point in 2016 too perfectly.
Also don't think Trump will (as effectively, anyway) be able to gish gallop his way through poor approval ratings this time because unlike in 2016, he's the incumbent and he has a political record. The fact that he was running as the insurgent in 2016 allowed people to more or less wishcast their ideal positions onto him, and that just can't happen this time. He's been doing this for three years now, you can't pretend he's going to enter office as some kind of rational centrist. Like I pointed out, swing voters thought Trump was closer to the center in 2016 than Hillary, think about that for a moment. 90% of this is projection.
Again - could very well be wrong on all of this! Let's just not pretend like "Biden will definitely lose" is anything more than a gut feeling.