Does not matter. If Trump is not running again, no matter how politically active he is that would not stop DoJ prosecuting him and it should not.
You are simply arguing that Trump as ex POTUS would be a better voice than Roger Stone or Bannon or others because he is an EX POTUS, but it is not the 'effectiveness of the voice' that matters. That is protected via the 1st amendment.
That is very different than an AG launching investigations against someone who is a declared candidate against your boss, the current POTUS.