Here’s a fun counterfactual. Suppose Spiro Agnew had not taken a plea in 1973 and decided to fight the criminal charges against him. He may have become President. Nixon resigned less than a year after Agnew did. At that point, the criminal case against Agnew would have presumably been put on hold because we don’t think a state prosecution can go forward against a sitting President.
Of course, Agnew could have been impeached. Or Nixon might not have resigned if Agnew was still in office. But for the 25th Amendment, Watergate would have been much worse.