Parties almost never alternate one term Presidency's, only time in American history was 1888 and 1892. 1896 started a new majority and Teddy broke it open for the GOP after 1902. You can sort of see this now, modest Biden/VP win this year and 2024 followed by GA, TX, NC, FL breaking open for Dems due to demographics in the middle of this decade.
We live in strange times, and I have little doubt that if Biden is elected he will only serve for one term. Biden will be nearly 82 years old by Election Day 2024. I think the bigger question is who becomes 47th president — a Republican or a Democrat? That's hard to say at this point, but we do tend to alternate.