The danger, as usual for Democrats, isn't people flipping to Trump, it's people not bothering to vote (or giving it to another party like the Greens) because they don't like the Dems' candidate. For the most part, Hillary didn't lose in 2016 because people voted for Trump instead of her; she lost because voters who normally would have voted (D) either stayed at home or voted for a protest party. Biden didn't have so much of a problem this way, for various reasons (voters liked him better to begin with, and they had just lived through the horrors of 4 years of a Trump presidency).