While I agree with you about swing voters, there is a very large number of potential non-voters, sometimes known as the enthusiasm gap. People that believe that both options are the same, or that neither will affect their lives very much. There's also the ones that think Trump can't get re-elected, which of course is part of how he got there in the first place.
There aren't a lot of people excited to vote Biden. But we'll do it anyway, because we really don't want to see more Trump. If I didn't see Trump as a threat to democracy itself, I could understand people not voting at all.