The problem I noted by just thinking of what they're asking for is that such a system substantially complicates the Ninja rotation, as now you have to factor in trying to land specific points in the combo before activating specific mudra that specifically boost that attack. The Mudra system seems like it was never designed or intended to be used to boost the next weapon skill used in quite this fashion, and the overwhelming amount of new abilities the devs would need to account for in such an addition would border on the insane when you finally got down to it. Even if it was just 3 new abilities that augmented your next weapon strike because you didn't fire it, what would be the fundamental difference between that and activating a specific GCD that would do the same thing?
In a system that by design mutually competes with literally everything else in the same system, it's a lot of extra bloat that doesn't really add much flavor when you truly delve into what is being asked for, and complicates a class in a way that it doesn't need to be complicated. Hell, as much as I hate positionals, it'd make more sense to make specific mudras fire when doing a specific positional than to tack on more features to the mudra system that it really doesn't need. For instance, side-stabbing someone with aeolian edge? Congratulations, have a fireball to your face! Backstab with Shadow Fang? Icy feet or what have you! It would fundamentally add the same thing without adding more competition to a system that is innately designed to compete with itself without ultimately devolving the system into "We don't actually know what half these abilities do because only these ones are ever worth using." Much like Hyoton, an ability so little used that you probably had to look it up simply to know what it was. Its only crime is that literally everything else is vastly superior to it in every way, shape, and form, unless you specifically want the ability to root things. Hell, even the bunny rabbit is vastly superior, because it does a dance and a jig to calm you down after messing up another ability for any given reason.