I understand Tobiis desire to break out of the niche and make eye tracking more mainstream. But why does this have to go at the expense of all the motivated developers here? There have been so many threads on programmatic calibration and programmatic enabling in recent years, but never a thread about removing API functions.
There is the TOBII_STATE_EXCLUSIVE_MODE. One possible solution could be that the critical applications you mentioned can launch the tracker in an exclusive mode and no other app can turn off the tracker in this mode. I have never seen a setting where a customer uses two different eye tracking apps at the same time.