It may be true that, in the past, if female athletes had not been separated from male athletes, we may not have heard of them. But today, things would surely be different. Females buy sports clothes and equipment too — so I imagine Nike and other major brands would still be very much interested in sponsoring female athletes, if we now had mixed-sex sports divisions.
Furthermore, if people who claim to be interested in women’s sport would no longer watch women playing sport if we had mixed-sex sports divisions, then what does that say about their claims to be supportive of women’s sports?
In any case, no sportsperson is entitled to our attention. We get to make our own decisions about which sports and which athletes we would like to pay attention to — and that’s how it should be. And if people want to watch the best sportspeople, regardless of sex or gender, there’s nothing wrong with that.
And ultimately, it‘s a point of principle. There are many ways a man can be disadvantaged in sport. He might, for example, be short. Or he might have been too poor to afford coaching. But if he’s worked just as hard as Serena Williams and can play tennis just as well, it’s sexist to say he can’t play in the same tournaments as Serena Williams, just because he is a man.