This isn’t really a tradition. The only time it became a live issue was Obama’s declining to go after GW Bush and Dick Cheney for war crimes in Iraq. e.g., pursuing an aggressive war, torture, etc.
In the case of Trump, the crimes are so obvious, so flagrant, and so numerous, that refusing to prosecute will send a message to all future presidents: “You can commit any array of felonies you like while you’re in the White House, and leave with impunity.”
Now, no one in America’s future will be nuking France just for fun, but we have just seen how dangerously close we came to the complete breakdown rule of law, and the seizure of absolute power. Failure to prosecute the many crimes surrounding this attempt to hijack the presidency would be a terrible mistake with permanent consequences.