During the soft retreat for 30 seconds, they heal to full health.
And you know the OKW counter can't retreat either?
It's very nice of you to stop bombarding it while your opponent repairs the non-functional mortar pit. And the OKW counter has the rather significant advantage of being able to move.
It is rather funny that people complain about brace when it's rather close to what entrenched positions were actually like. WW1 had taught that artillery was unlikely to actually destroy entrenched positions (and boy did they try). Instead arty bombardments were used to suppress (i.e. force them to brace) and follow up with an attack before the positions could be re-established.
In game there are more than enough highly effective counters to emplacements.