Presidential terms are a slow moving phenomena. For example, the pattern of eight-year presidencies you cite is actually two in a row. I remember similar comments be expressed back in the early eighties - claims that we had entered an era of disposable presidents based on the fact that two presidents in a row had not been re-elected.
If you look at a larger sample, such as the last ten presidents, you’ll see five were re-elected, three were not re-elected, one was both, and one was neither.