It was already proven to be inferior long before that. It's difficult to use in rough terrain and it's less mobile and versatile. Units cannot be broken off and retasked as easily and formations can't react and change direction as effectively. They were also fighting Goths with their heavy cavalry which historically didn't go well for their infantry in general which were surrounded and would not have gone well for phalanxes either as they are even easier to surround and flank.