JD, I disagree. You are still thinking about one aircraft. If you leave the first a/c going slow, you HAVE TO SLOW THE NEXT (and all subsequent) a/c, which puts the next a/c in an inefficient state. Your (very good) description of what happens when ATC fiddle with a descent (by applying speeds) also assumes that the descent has already commenced. A/c armed with the knowledge of what speed will be required on descent can adjust TOD, profile etc accordingly to achieve a better outcome. Conversely, if ATC had KNOWN in advance that this 777 was going to descend at 250kts, Im sure there could've been better planning, and a different result.
None of this addresses runway utilisation (as someone alluded to earlier), and the effect of under-utilizing the tarmac that slowing everyone down behind the 777 might've had (burning the slot you would've expected the 777 to fill).
When oil gets back to $150/b maybe someone will make a lot of money designing equipment /software that takes all the factors involved into account (already have/has?)?