Why can’t each aircraft quote a GPS distance from a location... blah blah
Well indeed. I am not aware of any such arrangement around the world, if somebody has news of one I would happily fight to implement it, but I don't see how you can get anything useful from distance reports between two aircraft that have not yet passed.
They are used all the time, with greatly reduced separation, for aircraft tracking in the same direction, (tracks different by <45 degrees), or aircraft that have passed, as I indicated...
...or wait til you've passed (plus variegated procedural standards).
If not, in the scenario above, the ADS/B equipped aircraft would have to wait 10 minutes AFTER the estimated time of passing to go through the clodhopper without the gear if he could not get above by 10 minutes before! My 80NM was a bit optimistic. If you figger 300kt gs for both it would be more like 100NM, increasing to 200 if unable to establish passing somehow. This is how it is done now.
On the other hand, the same direction RNAV standard is 30NM, with work-intensive voice distance checks to monitor two aircraft. If the same pair of aircraft had the gear to facilitate surveillance by ATC they could fit another 9 aircraft between them (theoretically
), and separate just by looking at them while eating a sandwich, (sandwich not compulsory...)