Sure, but in 3e/3.5 the abilities which the subclasses (Ranger, Paladin, Bard, Barbarian) had were generally weak in comparison to the abilities that they shared in common with the fighter. What was most important was the shared basic attack bonus and the shared feats (whether they got them as class abiltiies or not). The smite and healing of the paladin, and the spells and skills of the ranger, were both minor abilities that were tacked on a fighter shell.
Specialities can certainly carry the weight of those minor abilties, to the same degree that they are carried in 3.5. If it was good enough then (and still now given the amount who play 3.5) why isn't it good enough until we find something that is more robust and interesting as a mechanic for paladins and rangers?
How so? The specialties we have don't provide anything like those bonuses. Smite doesn't require taking a turn to buff up your weapon; specialties do.
Dual weilder: after two feats you do the same damage and have the same AC as a guy with no feats, weapon and shield. The only benefit so far is potentially critting more, and being able to spread damage around more flexibly.
Archer: pretty much the same, except actually good for rogues. You can ignore cover and trigger sneak attack if you have it.
Acolyte: the level 3 takes an action to swap damage types that can be cashed in for a reroll.
Etc. Survivor, Healer, and Guardian are the only things providing broadly useful options at this point. The two that can provide advantage are good for rogues.
Basically, there's CONSIDERABLY more differentiation at levels 1 - 3 between even related classes in 3e. A paladin has an actual smite, healing, fear immunity, aura of grace (one of the defining features of the 3.x pally IME), etc. The specialty/theme provides nothing in comparison.
Classes are more than a chain of weak feats. I mean, no one is saying that the wizard should be a skilled rogue type guy who takes Magic User to get spells. The spells provided by the magic user specialty are a wimpy version of the magic an actual wizard gets, even at level 1. It's the same thing for all the classes.
Even if the specialties were twice as good, they still wouldn't be adequate to recreate actual classes.
It's kind of like how classes like Duskblade or Magus, by having features that allow for ways to combine spells and fightiness in the same round, offer something to fighter/mage types beyond sticking a few weak spells on to a weaker fighter frame.