But it does make it more likely that more optimal routes can be offered, saving a few tons of fuel every day.
Which then get wasted flying in circles awaiting your allocated landing slot...
RNP/ADS-B/CPDLC/ADS-C/WAM/ADSB-IN etc. are all wonderful tools heading towards enroute free flight and better performing nav /comms systems; but there still is no real point if the bits of TAR are all full; generally speaking airspace isn't congested it's airports (admitting there are times when routes fill up)
I'm interested in what FEDex are doing re self spacing for landing (by airborne systems)? Is it working for them are they realising any true savings; or is it just smoke and mirrors? Are any others doing this?
Until you get fleet-wide systems in place there are massive limitations on this self separation, it only takes a few airframes to disrupt the whole show (like one non RVSM approved a/c flying in RVSM airspace)... Then there are the failure events....
I used to do APP control with RNP approach paths (apparently huge benefits for fuel burn and safety margins), the speed variations even with same type on the same route were huge (as the individual box is picking the individual optimal performance), combine that with different types on different approaches to the same bit of TAR and it becomes very inefficient because the safety margins and missed approach procedures were totally different and it was very difficult to judge what space and speeds between aircraft were required to enable two or more successful arrivals; so some flying the RNPs may have saved some juice, but others were cost that saving and probably more because these things exist...