This is the big reason I also don't like the addition of positionals to the Ninja class, though it's not nearly as profound as turning the most mobile class in the game into a class that treats the least mobile as company, even if it might not be as immobile as a Black Mage ultimately ends up being.
Classes that get their uniqueness expanded upon with new content? Fantastic. Classes that get something fundamental to the class substantially altered if not outright removed? Not fun at all. Though Bards definitely got hit harder than pretty much any other class.
Must every change be so fundamentally game-changing? The devs could have just as easily delayed the Ninja until Heavensward and people would still think it was awesome, despite largely being rather reserved as far as class abilities are concerned. About the flashiest thing the Ninja does is summon a single bolt of lightning. A far cry from the star-swirled sky that Arcanists get at level 50, nevermind the Nova that Black Mages summon. Looking at the preview, I was pretty happy watching the Ninja jump around the mob to hit it in multiple places. It's a sort of reserved flashiness that fit in just fine for the class.
It's no Akh Morn like the summoners get, but the Ninja was never as flashy as even the Monk or Dragoon is, and there's nothing wrong with that at all.
Furthermore, you're short-selling the utility of threat-manipulating abilities. For all we know, the devs are going to bring threat much, much closer together compared to now, or maybe the devs will add major threat wipe or threat jumbling mechanics that could make Ninjas a very valuable group member to deal with situations where the Healer was given phantom aggro. Sure, the tank could simply cast provoke, but sometimes having a little bit more is quite useful.
The class by design simply by proxy of Goad, nevermind Trick Attack, was always going to be a support DPS class, and having more support is fine by me. Nothing can be more useless than All Fours is, and it's not like Ninja is unique in receiving abilities of dubious worth. I still struggle to find use for Foresight on my Warrior, its effect is simply that intangible even on high-end raid content. Nevermind the threat dump on the Dragoon, which I never found a use for but have seen others use it for reasons other than a threat dump.
But do remember, a 4th mudra by itself, without altering anything about how mudras work right now, is adding seven new abilities to the class. The devs would be better served making a special mudra that, when combined with a very specific combo of mudra of N lengths long, would give the ninja some hyper-powerful buff or damage ability or something. That's something the devs could do that would be interesting and really feel awesome. But I digress.
At the end of the day, the devs did expand a lot on the Ninja with stuff we know, even though it's not simply a major mechanics overhaul like so many classes needed. The only problem I have is that the Ninja is gaining positionals. That's it. Everything else sounds good.
But I digress.