This is how I would expect maternity coverage to work: It will increase the annual cost of Ms. X's contract. Therefore, shoe company Z will offer a lower cash payment to Ms. X, but include maternity coverage. Ms. X won't actually "gain" anything.
If Ms. X is a "must have" athlete, and some are, then company Z will pay what it needs to pay.
But it will then lower its budget for other women athletes.
In this scenario, Ms. X does do better, but only at the expense of other women. The rich get richer; the poor fall farther behind.
I seriously doubt that shoe companies increase their overall athlete-promotion budgets every time some athlete comes up with a good reason--whether pregnancy, other health issues, rent, education, unemployment compensation, retirement funding, etc.
I'm in favor of societal support for all the above. But I'm dubious that we're near a historical point where employers are going to provide them to independent contractors, ie, freelancers.