There are certainly enough female baseball players of quality throughout the United States to justify a professional women's league. It would also be an important step forward in terms of gender equality in sport to give women their own professional league to play in. Female athletes rarely reach the same level as men in terms of success because the opportunities are simply not open for them.
Not only that, but it is about time that females where allowed to play in the regular major league sports. Sports should not be segregated by sex or gender, as they are able to compete against one another and there should definitely be more co-ed sports. It is time for a change.
People watch professional women gymnasts because they are good. They are much better than male gymnasts. Women's basketball isn't that popular because men are better at it. The same is true for female baseball players. On the whole, men are better at baseball, so that is who people want to watch. It's not sexism, it's based on performance in the sport.
As the question says, one woman was that good. It would be difficult to find a whole league at that talent level. The woman in question would make Pedro Martinez in the 90s look small, dominating the league due to the massive skill gap. If more women played baseball instead of softball at a young age, it could happen, but not as it stands now.