Yes you are correct in that the incumbent does not lose their parties nomination (looking like only 1 time in history with Franklin Pierce in the 1850's) BUT on the flip side of this if you are using history as a precedent, the previous oldest president at the END of their term was Reagan at almost 78 years old. Biden would be 86 if he wins reelection........so throwing political affiliation aside, it is clear Biden has some mental/cognitive decline, which is nothing against him, he is old and doing the most stressful job in the world, so I would think if the incumbent were ever to lose the nomination this election would make sense as I do think its fair to say Biden will not be in the correct cognitive state at 86 to run a country.
Again, just personal opinion for me, I just feel if the GOP was in shambles, had a weak candidate no one backed (or multiple splitting them up, etc) and it appeared it would be an easy win for the Dem's regardless of who they put up that Biden would remove himself from the ticket.
Just my opinion (and sadly it appears neither of us will find out who is right because it appears Trump will be the GOP choice)