The thing I keep hearing is that Biden steps down in like 2022, putting Kamala Harris in as President (and if it's before the mid-terms, Dem-controlled Congress can confirm her VP selection readily) and setting her up as the incumbent. I may be reading my own preference into the narrative, but I have always felt like Biden started his 2024 run because he really felt like the best (and maybe only) shot at beating Trump, and wouldn't have run if it was Romney or Jeb! or some other relatively moderate Republican incumbent.
And how does an incumbent president being defeated figure into the historical trend? And how about the Trumpist half of the GOP appearing determined to primary the moderate Republican incumbents that haven't already stepped aside at Trump's own behest, as even having a politically active former president immediately after the election in which he departed DC is something of a historical aberration.