It would be one of many possible features firms could compete on. Developing more accessible apps would result in an increased userbase leading to greater revenue (and potentially take you from a perfect competition to monopolistic competition secnario, reducing the elasticity of demand for your product, allowing you to raise the price).

> Developing more accessible apps would result in an increased userbase leading to greater revenue

Only if the marginal cost of developing them is less than the revenue attained by developing them, which is also limited by the amount of funds that disabled people have.

Disabled people existed before ADA, and the reason ADA was popular was because it didn’t make business sense to serve the disabled population. There’s no way construction costs of ADA accessible features of a building will ever be recouped by sales to disabled people needing it.

A free market solution would require the government to give sufficient cash to disabled people, enough to make businesses want to compete for them. But that is also wrought with possibilities for corruption.