LHR
I know what you are intending, the issue is surely how to program a system to recognise the variables associated with
a computer program checking an IFR aircrafts position against protected areas ( LSALT, Grid, MAS, MVA, Approaches)
Then we need to define a 'grave' error in altitude and apply that algorithm to the mix. At this point it is becoming reliance on an ATC getting alarms via a complex program distant from the real action.
Yes, many IFR aircraft are not TSO 146 equipped. The JCP subsidy (Navaid component not ADS-B per se) was to so equip ALL. I believe that alters the equation and places control where it belongs - the person who loses if they make T28's 'grave' mistake, not the ATC.
The other issue is radar and ADS-B coverage. It wasn't designed for cover to ground except where it has a clear view. Probably more effective cover with ADS-B but still not 'global'.
I understand the concept of automatic. I also understand why some alarms are turned off. I believe a lot of VCA could be prevented if the CTA step alarms were enabled - problem is picking an emerging VCA from the other 999 aircraft flying close enough to activate the alarm - but not actually penetrating.