>16% of people have some kind of accessibility requirement
16% is 4 in 25 people, also no guarantee you'll be able to accommodate their specific accessibility requirement, depending on the app, and depending on the requirement.
That's not to say the app shouldn't be accessible, more that, you should think about when you spend the money to implement things like this, and how that money may best be spent to accommodate the highest number of people. Figure out who those people are, then figure out what your revenue growth would be if you captured those people, then compare those numbers to how much you'll have to spend to accommodate their needs. That's no different than any feature upgrade really.
Where you'll likely arrive at is the same place many apps have, unfortunately, arrived at, which is to probably just get text scaling quasi working and leave it at that because anything more is throwing cash into a black hole.