You should never say never but IMHO Bernie Sanders will have a very difficult time unseating Donny if the current economic and social conditions hold.
According to Gallup
90% of Americans say they are satisfied with their personal lives (January 2020), and when Gallup broke that down by different areas of their personal lives (2019) the numbers are equally overwhelmingly positive.
Satisfaction With Personal Life
Given the positive feelings that the vast majority of Americans are showing with the status quo, why would they vote to replace the POTUS with a radical 78 year socialist? I could see this as a possibility if we were back in 2008 in the midst of economic and social turmoil, but now?
What strategy would a Sanders campaign pursue? "
I know you think you're satisfied but you're too stupid to know that you shouldn't be" ?
The wildcard factor is of course Donny, being the pompous ass that he is, doing something idiotic to beat himself but barring that or a complete economic or social meltdown, I'd put Comrade Sanders chances of winning a General Election in the slim to very slim category.