Can you imagine the Framers foreseeing a case where someone would be impeached after leaving office? (Of course the purpose was to try to bar Trump from ever holding office again.)
John Kerry gave aid and comfort to enemies of the US (by going to Paris to give advice to the North Vietnamese and Viet Cong delegations) but no one used that to keep him off ballots in 2004. And he very nearly won (the Ohio electoral votes made the difference).
Yes logic and fairness are rare in politics today. We are in a no holds battle for power and one side in particular is willing to use almost any tactic to get that power. Unfortunately they have legions of adherents who think it is perfectly acceptable to “do whatever it takes” to meet their goals.