Walking is why we have mounts like chocobos and coeurls, to carry us faster than we can walk. Sprint would invalidate mounts, but Sprint only lasts for 20s on a 30s cooldown relegating it to 'gotta go fast' moments. Flying using personal wings, though, would invalidate flying mounts as why use them if I can do it myself with potentially none of the drawbacks of being on a mount (i.e. being able to attack and interact with the environment). Now if you had wings work LIKE a flying mount then maybe... but then you'd probably have those that would complain, "Why have wings if they're just like if I was using a flying mount?!" Which I would agree to an extent, why have them at all given that having them work as if walking would make them overpowered?