[gjhan]

Your point that speeds are restricted to be lower at lower altitudes is really salient to the discussion because it's an intentional procedural mitigation for collision hazards in the more densely-populated and less-controlled airspace down low. Drones were but a glimmer in anyone's eye at the time the regulation was emplaced, but the same theories are still relevant.

Bear in mind though that the under-10,000 ft limit is knots of "indicated airspeed", which is a constant-dynamic-pressure pseudospeed uncorrected for density. 250 KIAS at 10,000 ft is about 290 knots/330 mph relative to the airmass, and sometimes up to 10 kt faster on a hot day.