I know it is anecdotal evidence, but really, who knows anybody who voted for Trump in 2016 and now will/has voted for Biden?
I have zero friends and acquaintances that have said they are changing from 2016, granted I live in a pretty deep red area. But direct family, wife/kids/son in law, in 2016 we were 3 MAGA votes, one libertarian, and one no vote. And my daughter who did vote Trump in 2016 made up her mind waiting in line to vote while talking to my wife, and cried because she told her boyfriend/now husband she wouldn't. Now in 2020, we have 5 straight down the line republican voters.
I'm sure there are some people who voted for the President in 2016 and are not going to this year, but I would guess they are swamped with a tsunami of people switching to MAGA and new red voters.
So, how does Biden win? Massive get out the vote enthusiasm like 0bama had, or fraud, those are the only ways. He has only Orange man bad enthusiasm going for him, and negative enthusiasm doesn't bring out the votes.
Anyway tell me your first hand stories of vote switching this year, I really think GEOTUS will pick up several percentage points of switchers in 2020.