Personally, I can't wait to see the CR 19-22 stat blocks for the demon lords. Now, I agree that in any campaign I run, the demon lords are going to be advanced. I won't use the stat blocks as written.
For them.
But damned if they won't make for some excellent demon adversaries in their own right. A name change, a slight tweak in description, and bingo. Got a brand new baddie for most campaigns, and the option to advance them if I ever have the need to use a "real" demon lord.
(Plus, of course, there's always the aspect/avatar option, which I can also see myself using pretty frequently. But that's already been covered.)