They did try in Stormblood to make stances more relevant... but it didn't really turn out very well. Problem is that with three options, one will always be superior. And for monks, that option will always be the one that raises your damage output the most. Even if they all did similar things, like one gives flat potency buff, one raises crit rate, and one reduces GCD, people will math it out, figure out which one gives you the most advantage, and that will be the one and only stance you use.
This was a thing we saw with Scholar too. Selene was widely considered to be vastly superior to Eos, and now both the fairies just do the same thing as each other.
