If Apple were to add new APIs, it might be possible to use personal cloud storage (NAS, Decentralized Web Nodes, etc.) with the same UX as iCloud with E2EE.
Which would quickly become illegal if UKGOV is set on getting access to people's iOS backups / cloud storage / etc. Hell, it's already a legal requirement to hand over your keys if UKGOV demands them[0].
[0] "Regulation of Investigatory Powers Act 2000 part III (RIPA 3) gives the UK power to authorities to compel the disclosure of encryption keys or decryption of encrypted data by way of a Section 49 Notice." https://wiki.openrightsgroup.org/wiki/Regulation_of_Investig...
Half the computer crimes in the UK involve illegal access to the PNC (police national computer), how exactly do we think this would go.
For all the checks you put on people who can access this stuff the temptation is too big - just look at the intelligence analysts using systems to stalk Exs etc.
For any system like this to exist you must ask yourself if you would be happy with the worst person you know having a job where they have access to it.