“Beating an incumbent is hard: Ronald Reagan, George W. Bush, Bill Clinton and Barack Obama won their reelections not because they were great presidents but because they were incumbents.”
Valid premise - beating an incumbent IS hard - but poorly argued when two of your four examples, Reagan and Clinton, DID beat incumbents.
The point is, the incumbent is highly favored to win, all things being equal.
All things were not equal in 1980 - Reagan ran circles around Carter. That wasn’t even a fair fight.
All things were not equal in 1992 either, but in a different way. Clinton won with a mere 43% plurality, because third party candidate Ross Perot’s 20% of the vote were pro business Republicans, who would otherwise have voted for Bush. Without Perot, it might well have been 67/43, an epic popular landslide for Bush.
Nobody could seriously argue that Biden vs. Trump in 2020 bears even the slightest resemblance to Reagan vs. Carter in 1980. More like Mondale vs. Reagan in 1984.
Likewise, there is no third party challenge in 2020 the likes of Ross Perot.
Ross Perot ended up being a useful idiot For the Democrats, doing great harm to this country and esp minorities.