I'm still far from rules mastery, so forgive me if there's an obvious rebuttal to my thinking here but...with the way hit points work, while Con may be better, Dex seems to be less onerous than you insist, considering that with Dex or Int you also increase your Reflex Save.
It's all a matter of choice.
Sure, one could have Plate and a good Dex for Reflex. In fact, many sword and board PCs would do this so that they could garner Shield Specialization.
But, how does one get both the best AC and the best movement?
Answer: One cannot. It's one or the other.
The best AC is Plate and Plate (or Shield) Specialization and Heavy Shield.
The best movement with a good but not best AC is Scale and Scale (or Shield) Specialization and Heavy Shield.
You are proposing to combine the best movement with the best AC so that there is never any reason to take Scale Armor Specialization.
And you do not see that as imbalancing?
Shouldn't an extra feat garner an extra benefit? I don't see the reason why spending an extra feat for plate garnering more benefit is a negative?
One does garner extra for Plate Proficiency (better AC) and Armor Specialization Plate (better AC).
The reason Armor Specialization Scale works the way it does is because otherwise, nobody would take that. Everyone would be in Plate.
It's a balance issue. With your proposed rule, nobody would take Scale. With AS Scale not having the extra bennie, nobody would take Scale.
It's written the way it is to incentivize players to use Scale armor. Yes, the feat is better than the Plate feat. But, there is a definitive game balance reason for that.
Who said that all feats must be equal?