Isn't it bit ironical or naive to trust their current software as it is (which is almost full blackbox), and then speculate what they could do without saying, when they add something?

As far as I understand, you can disable this feature, because it is tied to iCloud sync. Based on their spec [1], this feature avoids to do the same as Google and others doing (scan everything on cloud), instead they scan on device, which limits exposed data what Apple sees. So this is improvement compared to other available solutions.

[1] https://www.apple.com/child-safety/pdf/Expanded_Protections_...

Exactly. Many people who are upset about having their images scanned before going to iCloud don't seem to realize all the big providers (Apple, Google, FB, Twitter, MS, etc...) have been scanning images with CSAM for years already.

The client side/server side also does not matter because iOS users have had to trust Apple implicitly since day 1. All the 'what ifs' existed whether or not Apple added this feature.

I speculate that Apple is going to announce an expansion of E2E to more services at the iPhone event this year, and this feature is getting in front of political complaints that could lead to real privacy destroying legislation/LEO complaints.

This makes the most sense for “why” and “why now” IMO.