The terms 'scandal' 'affair' 'incident' and 'crisis' were widely used, but there wasn't a journalistic convention until Watergate.

This is kind of an odd question. I mean the creation of a word or suffix does not require an existing one to be replaced.

In terms of U.S. politics I think you'll find most controversies ended in "scandal," as in "Teapot Dome Scandal" or "Star Route Scandal" etc. If we conclude that "scandal" is no longer the common labeling vs. "gate" then we can maybe conclude that "gate" replaced "scandal."

There would not have been one. The Watergate scandal was so named because the break-ins occured at the Watergate Complex. Other "-gate" sandals were given this name to allude to the Watergate scandal.