Notso
A CDA actually applies from 6000' (see the definition I gave a couple of pages ago) in the Heathrow case.
I don't know why you think it is impracticable from higher up - we do it every day, holding or not. 757/767 - programme the FMC with M0.8/290kts in the descent page, standard height constraints on the way down, 250kts FL80+ 12nm before the fix, 210/220kts FL80 at the fix, start the descent a bit before the machine tells you to - works a treat!
(I do agree it can't always be done, but themwasthe days seemed to be looking for ideas and I felt that Heathrow was a good a place as any to start).
G W-H