Speaking for myself, I am on iOS 15 and don't care all that much about the CSAM feature. That said, I can see why others would assess the small potential benefits from catching people dumb enough to upload illegal content to iCloud and find those benefits lacking compared to the risks of giving Apple backdoor on-device access. Saying "If you have nothing to hide" in conversations about respecting privacy is literally the most cliched and thoughtless approach an individual can take, and I hope you take some time to reflect on why you would gravitate to that talking point.
Why are you worried about a (non-existent) back door when this only comes into play if you open the front door? Right now, Apple has the cryptographic key to see every photo you upload onto icloud. Wouldn’t you prefer that they did not?