Without Watergate; there would have been no Gerald Ford incumbency to block Ronald Reagan from getting the nomination in ‘76.
Yes, but then you would have the historical precedent of it being nearly impossible for a candidate from the same party to follow a two-term president into office. Bush Sr. was able to do that only because Reagan was so popular.
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Then he could have gotten hit with all the catastrophes of the 70s - inflation, recession, oil crunch, Iran, Afghanistan - instead of Carter. And it probably took time for Reagan to work out national policies that fit the times. Reagan in the Seventies hadn't yet put together the team and the ideas he'd make use of in the Eighties. Without Watergate, there would have been no Carter, but maybe a more competent Democrat who would have stuck around longer. Or maybe Reagan (or another Republican) would have won and gotten swamped by the messes of the Seventies.