I think this is a really important point. Women's tennis is as popular as men's tennis. If a world-class female player played a world-class male player, the woman would get crushed. People are perfectly happy to watch tennis games that they know are not between the Best Players in the World.

If the best three or four women left women's tennis to compete in men's tennis, women's tennis would suffer badly. Why the hell would I watch women's tennis knowing that I wouldn't see Maria Sharapova?

(Admittedly I'd pay to watch Maria Sharapova set a thermostat, but my main point is still true.)