It's a REALLY BAD precedent to set having new Presidents go after old ones. That's why they avoid it at all costs. You don't want to turn the US into some junta where the next President throws the guy he beat in an election in jail.
As for Trump, he's still fukked because the shyt he's going down for happened *before* he was President. And once the black-and-white evidence is out and there's no Barr to protect him, it's over. Trump right now, TODAY is an un-indicted co-conspirator in at 3 crimes that lead to guilty verdicts.
* Trumps fradulent charity he stole from for personal use
* Trump University fraud
* The Stormy Daniels debacle with his lawyer. If Cohen was guilty then Trump had to be as well.