Should Joe Biden be prosecuted for negligently storing classified documents in his home? Every person in the US is guilty of some baseline level of criminality, and the more “interesting” your life, the more crimes you are likely to have committed.
If the US wants to have non-criminal presidents then it should reform its vast state and federal criminal codes so that such a person can actually exist. Until that happens, a norm against criminalizing ex-presidents (except for serious crimes) is a good norm to have.
[I’m not sure it has to be said here, but my political beliefs are completely disjoint with Trump’s]