To be fair, some (automatic) vehicles have such tall gearing that coasting will not slow you down if on a flat, let alone downhill. I've driven a few, and I can't stand them. You have no choice but to tap brakes.

I much prefer cars with short gearing for better engine braking. I continue to choose to drive a manual party for better engine braking.

So many people don't even use regular cruise control. They'll have nobody in front of them for miles and their speed yo-yos between 60 and 65 mph. And if you pass them because your cruise is set at 66 mph, they'll speed up to match, but then eventually decide to pass you, get in front, and slow back down to 60. The only way to end the stupid cycle is to go 70-75 mph until there's at least 1/4 mile between the two of you.

and there is a huge overlap between those people, and those who slow down to 60 to save on fuel.

If they'd just drive 70 on cruise control, they'd be faster, less traffic jams and less fuel use...

I really would like to see some mandatory use of cruise control on some stretches of highway, even if just as an experiment

The adaptive cruise control in my Subaru rarely coasts. It isn't smart enough to see a gap slowly narrowing and start coasting, and it isn't confident enough to temporarily allow the gap to be smaller than the setpoint while it coasts to recover the gap. So it brakes and wastes energy, and I don't use it.