Presidential impunity has repeatedly failed
By 
Phil Lacovera, the counsel for the Watergate special prosecutor (who, it happens, I met a few times when we were both teaching at CUNY), argues that preemptive impunity for Trump would be a serious mistake:
After the final tumultuous months of the Nixon presidency, Gerald R. Ford decided to end the “long national nightmare” that was Watergate by pardoning his predecessor, thus sparing Richard M. Nixon from the dock where his senior aides awaited trial. Because I considered Ford’s pardon a serious mistake, I resigned in protest as the counsel to the Watergate special prosecutor. I hope that when President-elect Joe Biden assumes office, he will not repeat the same mistake.
[…]
After the final tumultuous months of the Nixon presidency, Gerald R. Ford decided to end the “long national nightmare” that was Watergate by pardoning his predecessor, thus sparing Richard M. Nixon from the dock where his senior aides awaited trial. Because I considered Ford’s pardon a serious mistake, I resigned in protest as the counsel to the Watergate special prosecutor. I hope that when President-elect Joe Biden assumes office, he will not repeat the same mistake.
As Lacovera says, the impeachment process having become a dead letter would make giving former presidents total immunity an even bigger mistake. Fiat justitia, ruat caelum.