A factor not often discussed is the random flying habits of poorly trained or under confident pilots.
Several carriers I have worked for are full of pilots who happily ignore ATC speed requests and pretty much do what ever speed they like on descent and will throw the anchor out at whatever point intrudes on their comfort factor.
The steely eyed missile men of ye olden days driving the 727 at 300 knots until less than 20 miles are gone, now replaced by kids who would rather stick like glue to an inaccurate DES profile and let the speed drift back to 260 knots than actually maintain an appropriate speed and run the numbers in their head.
Run the numbers in their head? Not anymore, no one teaches it. Many pilots these days have no clue about where they should be speed/height/distance.
Start training pilots a little better and forcing some flying discipline and things may work out a little better for ATC
Rant over.