Based on that premise, Biden would really be threading the needle (to use your own term). Its not impossible, but its exceedingly rare for someone to win a 2 party race that is based on the anti-vote. It may work out, but if it does I think we can expect way more negative politics than we've seen.
If this was a more divisive or base-driven candidate (like Sanders or Warren) then I'd agree with you more. But a "meh" candidate like Biden makes this a referendum on Trump more than anything else. Trump is the incumbent. Americans know what a Trump presidency looks like.
If you were to poll Trump vs "Generic Democrat" he's going to land underwater. He was like -7% on that kind of polling back during the primary season.