The point is that offline content detection on data that will be uploaded to iCloud feels like a prelude to offline detection of all content on your device.

In fact, given that Apple do not E2E encrypt backups and already implement CSAM functionality by scanning data in iCloud [1] it's not clear what the purpose of the new PSI/CSAM system is. Which leads many to speculate that it's only purpose is as a prelude to scanning all offline content.

[1] https://nakedsecurity.sophos.com/2020/01/09/apples-scanning-...

That’s not enough, Apple is “open” to adding these APIs to 3rd party apps as of today’s news.

It was never going to stop at iCloud uploads.

I fully agree but Apple is pushing iCloud hard. Hell it's seemless ecosystem is their main selling point.