Various points you seem to missing:
Traits are valuable
You only get two, and some of the options you can get for a trait are fantastically powerful. Reducing ACP by 1 is really minor.
Mithral is expensive
For a medium armor, it costs 4,000 gp; that’s quite a bit. Affordable by mid-low levels, sure, and quite cheap at mid-high levels, but for a while there, if you have mithral, you don’t have any +1-equivalent special properties (+1 something breastplate and mithral breastplate cost the same, after all). By the time you can afford a +1 something mithral breastplate, someone else can just about afford a +1 2-equivalent breastplate, which could be that much better.
Eventually, the fact that mithral is a static cost rather than a quadratic one as with most magic properties makes it a fairly cheap thing to get, but at that point you have to consider...
Making something from mithral means it’s not something else
There are a lot of special materials in Pathfinder; mithral’s a good one, but there are plenty of other good ones. Adamantine armor gives DR/– and is extremely difficult to damage. Dragonhide reduces the costs of adding energy-resisting magic to the armor. And so on.
3.5 let Mithral reduce the proficiency necessary to wear armor; it never broke anything
Rogues, along with bards and rangers, used to be able to wear mithral breastplates with proficiency, because 3.5 did not have Pathfinder’s clause that mithral doesn’t reduce the proficiency required to use the armor. This never broke anything; it was a useful feature but not an amazing one. In this respect, the fact that these classes can wear mithral breastplates with no penalty if they have Armor Expert is a loss for them, because they used to not even need Armor Expert.
You really aren’t getting that much return on your investment here
Attacks can target any of five stats: armored AC, touch AC, Fortitude, Reflex, and Will. This armor is effective only against one of those, and it’s a fairly modest bump in that defense (+2 AC vs. the more typical chain shirt for light-armor-wearing classes).
So you’re talking about 4,000 gp and a trait (and the inability to use a different material) for a modest boost to just one defense of many. That’s not outrageous at all; in fact I feel like that’s a pretty poor trade, overall. Especially when you consider that, on the whole, armored AC tends to be targeted by HP damage; no one wants to get hurt, but you have your HP there as a cushion to absorb a few blows. Meanwhile failing a single Fort or Will save can cripple or kill you, or worse!