And that's the thing. We don't want things to be complex for the sake of being complex, but fun. I feel that ARR (for the most part) has pulled it off really well with regards to how each of the classes reaches their role's goal (dps, healing, tanking) without straying so far that it encouraged class stacking for certain encounters. I'm all for all for having classes having different mechanics if they are designed with it in mind and it works. (stances, aetherflow, positional, etc etc)
As far as job changes goes, I've always seen them as a "promotion" of sorts to the base class (essentially adding new abilities, not changing their whole class dynamic). I don't expect a paladin to play significantly different from a gladiator when you need to be a lvl 30 gladiator to attain paladin and it runs off gladiator's exp bar. Though I'd personally like to see some more reactive skills on the tanks in generals, something to encourage actually using shield swipe when it procs. Whether or not they're pulled off well for the theming (looking at you, BRD and SMN) is another issue (which is never addressed by OP and only by you to an extent up to now, so it doesn't really fit in the grand scheme of this topic.)
The way I see the basic, barebones of a class, is what sort of role they fit (the dps, ranged dps, healers, and tank). What sets them apart for me is how they achieve that; DRG and MNK have incredibly different mechancs with stances and comboing, ARC has the management of aetherflow vs BLM's astral/umbral pendulum. That's touching into the whole subjective thing on what makes a class unique or not, and we'd disagree there (I feel that they;re unique enough that they can be considered different classes, you and possibly the OP since he just jumped onto your post without any other support feel that they're close, or maybe too close, to the barebone of the roles.)
Ninja is most likely going to play different from MNK and DRG anyway in the same manner (different abilities, mudras, concepts), but all of those that are in place already between the MNK and DRG (stances versus combos, positional, lance vs fisticuffs), which OP seems unsatisfied with so I'm not sure if ninja is even going to suffice for his taste.
Though from the couple of recent posts, there's really no more point to continue on. The OP is still out for blood and throwing out retaliation to everyone that challenges him, let alone refuting points.