Just imagine Germans had the "Krupp Steel" ability on their tanks that would make them take almost no damage from AT. This is what brace is. It counters the counter to emplacements.
And before you try to bring smoke into play: Any braindead can attack ground and still kill the tank + it costs ammunition + it´s locked in doctrines.
Brace is just that: Invincibility.
It doesn't quite work as a comparison unless your hypothetical ability also stops the tank from moving and shooting also. And it's still not a counter because the thing that threatens it will still be there after it runs out unless you properly counter it.
I think Brace is too powerful but it's also necessary to have something of its sort otherwise the emplacements become stationary death pits of doom.
Short of removal of emplacements, which is really not happening, the brace skill is needed, though it needs to be changed.
It might just need the damage reduction reduced a bit so it's destroyable with concentrated fire but gives enough time to counter the enemy before the emplacement dies if you have units nearby.