ATC Watcher:
On the 250 Kts below 10.000 , this is not to bother pilots nor a " power" thing for ATC . It is there for 2 things :1) giving time to spot and avoid VFRs (and give a chance to VFR to spot you) and 2) survive a hit from a bird in the windscreen ( Civil aircraft certification limit) This is a rule, written down in AIPs.
However, 250 below 10,000 wasn't always a rule. I can only speak to the U.S. When I went with TWA in January, 1964, their was no speed limit below 10,000 except within the old Airport Traffic Area (generally within 5 miles of the airport below 2,000 (or perhaps 3,000) agl).
The TWA/UAL mid-air over New York in December, 1960 finally worked through the regulatory system and sometime in 1964 or perhaps 1965 the rule was changed to 250 below 10,000 and within 30 miles of the destination airport.
Then in 1967 a TWA DC-9-10 was flying from KPIT to KCMH at 8,000. At more than 30 miles from CMH he was doing something near barber-pole. He overtook a Beech Baron and all aboard perished. That accident resulted in 250 below 10,000 in all U.S. domestic airspace.
The 1960 mid-air over NYC was in IMC, so it was not a see-and-avoid issue, rather more for more time to correct errors by pilots or ATC in terminal airspace.
The 1967 mid-air was about see-and-avoid.
Windshield limits were never a consideration in any of this rule-making, at least so far as I recall. There were a whole lot of high-speed operations below 10,000 from the advent of civil jet transports in 1958 until the present rule came about in 1968 (as I recall) from the 1967 mid-air.