That's the beauty of speculation. It might be right and it might be wrong, but it's sure is a lot of fun.
I take Biden at his word when he said during the primaries he'd serve one term. Now whether he meant that or not or just said that to attract a few votes his way is another matter. Almost all a politician, candidate says during the campaign, primary or general, one can basically just throw it out the window.
One thing to remember, having a president who has a primary challenge either ends up dropping out of the race of loses in the general election. Hence everyone doing his best to prevent a primary challenge.
Truman was challenge by Kefauver in 1952, Truman dropped out.
LBJ challenged by McCarthy, LBJ dropped out in 1968
Reagan challenged Ford in 1976, Ford lost to Carter
Buchanan challenged Bush in 1992, Bush lost to Bill Clinton
The thing is if a sitting president has a primary challenge, that sitting president is in trouble to begin with.
I'll stick with 2022, I think whatever happens in 2022 will directly effect the 2024 election.