> Why would any future president ever want to relinquish power, when a future of prosecution awaits him.
Because if he doesn't relinquish it the other branches will remove him anyway, as was imminent with Nixon. Trump wasn't convicted by the Senate, but his butt was still kicked out of the White House on Inauguration Day. American democracy isn't fantastic but it's still a long way from the President having absolute control.
"They will double down if we try to punish them." is a hollow argument that could be used to justify not punishing any crime. A pickpocket might try to run or fight back if he knows he will be punished, but that's not a reason to let him do it, it's a reason to have teams of fit people equipped with tazers and handcuffs that can subdue him anyway.