Well as Gary pointed out in one of the threads
Something to consider in selecting max speed is the energy of motion. There is a big increase in kinetic energy as speed increases.
A 5lb object traveling
@ 30 mph has 150.4 Ft Lbs of Kinetic energy
@ 60 mph has 601.7 Ft Lbs of Kinetic Energy
@ 90 mph has 1354.5 Ft Lbs of kinetic energy
What ruler-makers may consider is the potential result (of) an impact. Flying a model FPV doesn't change that potential except to expand the area in which it could occur from the immediate area around the pilot/flying field to whatever height/distance is defined as it's operating range.
Do FPV'ers fly in parks? Yes some do. However now 550 gives them access to fields. This allows the AMA to collect a bit of data on the subject. That data can be presented to the FAA giving key points on FPV's impact on NAS.