The comparison is not yesterday's Centurion vs today's Centurion, it's Centurion vs cards from other issuers. Who is their competition? In many instances the competition is another American Express product, and in that circumstance they do not seem to be concerned if a cardholder downgrades to a different color.
This seems right apparently. I did ask last year on whether i can downgrade to platinum, but was advised that i have to re-apply for AMEX Platinum, and i couldn't just downgrade...
Still, i see that no one really mentions any interesting benefits around, so i guess we are stuck with what is there.
At least the US Centurion offers some better benefits than the IDC