It's not really correct to think of them as only differing by their energy state. The second and third generations of quarks (strange/charm and bottom/top) have

It's not really correct to think of them as only differing by their energy state. The second and third generations of quarks (strange/charm and bottom/top) have additional quantum numbers, flavor quantum numbers, that also need to be conserved. In addition, a defining characteristic of the strong force, as described by QCD, is the existence of the well known phenomenon called Asymptotic Freedom [1]. Based on the theory of QCD this is only possible if there are no more than 8 generations of quarks (16 flavors in total). Granted, we are only aware of three generations at the moment but this does put an upper limit on the total number of possible quark generations. I know this isn't the most satisfying answer to your question but I just wanted to point out that there are some well known differences and bounds on the number of quark generations. [1]: https://en.wikipedia.org/wiki/Asymptotic_freedom https://en.wikipedia.org/wiki/Asymptotic_freedom