FWIW, iMessage is largely unencrypted already. Yes, the messages themselves are E2E encrypted, but most people do not enable Advanced Data Protection, but do enable iCloud Backups, which causes messages only to be encrypted at-rest with keys that Apple holds [1]. And even if you enable ADP, most people that you communicate with didn't.
A similar situation applies to WhatsApp. On iPhone, the messages are stored in iCloud Backups. On Android they can be backed up to Google Drive but E2E encryption is not the default.
The only exception is Signal, which opts out of iCloud and Android backups and had to roll their own backup solution for E2E backups.
So in practice, US law enforcement can probably already access iMessage/WhatsApp messages.
A similar situation applies to WhatsApp. On iPhone, the messages are stored in iCloud Backups. On Android they can be backed up to Google Drive but E2E encryption is not the default.
The only exception is Signal, which opts out of iCloud and Android backups and had to roll their own backup solution for E2E backups.
So in practice, US law enforcement can probably already access iMessage/WhatsApp messages.
[1] See footnote 9: https://support.apple.com/en-us/102651