Ironically the only people who know about ADP and turned it on are Criminals.
It's still encrypted. Just not as much.
I bet about 0.1% of people used it... or even knew about it.
There are a lot of people who are not criminals who turn it on. Do you want everyone looking at every photo you've ever taken? The NSA, MI5/JIO, CCCPC, BND, FSB etc having access to everything on your iPhone? Some random person?
When there is ONE key (or one group of keys) that encrypt all the data held in Apple's iCloud environment (which in turn is held at AWS, Google etc, just encrypted - but then likely available to be copied by the NSA for example) many if not all of these agencies have copies of iCloud backups. They are encrypted in transit and at rest there, but these agencies (particularly the NSA) vacuum up all the data that they can and store it. After that all it takes is one corrupt employee, one blackmailed employee, one incompetent employee, one phished employee etc who had access to the keys to let them slip. And then everyone's data is out there. It is a huge target, one that any of these agencies or criminals would like to get ahold of and consequently there would be a huge financial reward to doing so.
The 3 letter agencies keep copies of it from 1, 5, 10 or whatever years ago so that when a key is leaked, they can get access whenever they want to do so. Whether it is for a true investigation, to leak something damaging to someone else (e.g. the tax returns from the IRS), or to look up an old boyfriend or girlfriend they'll have access. Think about journalists reporting on assassinations or something and say Iran gets their data unlocked. Or the US, or the UK etc.
If you have to attack each phone individually, it is much more difficult and much less of a huge target. If I can get access to 1 billion's people's bank password vs 1, which is a better thing to try? If I can get access to every actor and actresses photos (if they don't have ADP on) vs 1, which would I want? If I can get access to every CEO or other managers financial data a week before a company announces it vs just one, which would be a bigger target?
And there are hundreds of other scenarios similar to that - an abusive spouse, police investigations etc.