>Reason probably is that, at long distance, lower air resistance at height starts to count.

This isn't as big a factor as you might think. While older aircraft were optimized for design/cruise altitude, modern jets are designed to account for the imposed altitude restrictions and speed limits of departure and approach. They also have a much wider/efficient operating envelope, and due to improved logistics, are dispatched, loaded, and operated more appropriately wrt to flight plan profiles.