The Lefty O'Doul era had come and went. The Pacific Coast League continued to get by. Attempts to become a "third major league" came to nothing. Some teams became affiliates of major league clubs. The Oakland Oaks packed up and moved to Vancouver BC after the '55 season.
The monumental decision of the Giants and the Dodgers to move to California after the '57 season was the death knell for minor league baseball in SF. The Seals became the Giants' AAA club and moved to Phoenix, and over time have morphed into the Fresno Grizzlies.
From Wikipedia: "The Giants' final three years in New York City were unmemorable. They stumbled to third place the year after their World Series win and attendance fell off precipitously. While seeking a new stadium to replace the crumbling Polo Grounds, the Giants began to contemplate a move from New York, initially considering Metropolitan Stadium in Minneapolis-St. Paul, which was home to their top farm team, the Minneapolis Millers. Under the rules of the time, the Giants' ownership of the Millers gave them priority rights to a major league team in the area."
"At this time, the Giants were approached by San Francisco mayor George Christopher. Despite objections from shareholders such as Joan Whitney Payson, majority owner Horace Stoneham entered into negotiations with San Francisco officials around the same time that Dodgers' owner Walter O'Malley was courting the city of Los Angeles. O'Malley had been told that the Dodgers would not be allowed to move to Los Angeles unless a second team moved to California as well. He pushed Stoneham toward relocation. In the summer of 1957, both the New York Giants and the Brooklyn Dodgers announced their moves to California, and the golden age of baseball in the New York area ended."
Next: Opening Day 1958