It would make it very difficult to produce new CSAM if you eliminate the encrypted channels over which children are discreetly coerced into running away or producing it themselves.
Nonetheless I agree the utility that children get from access to the mainstream "Five Giant Enshitified Websites" doesn't really surmount the damage done. The main harm, in my opinion, is simply to general mental health and wellbeing. So that just adds to the case for an age restriction for smartphones. If that reduces potential contact with nonces then all-good, but the reality is that child abuse happens by people IRL, usually trusted family or neighbours.
Maybe the tide is turning and we're "getting there", and that's why I defended and supported many parts of the UK Online Safety Bill - even though other parts of it are trash. However this focus on CSAM to push unworkable restrictions is the usual dishonest politics and posturing.
We have more general problems with minors and the Internet, and most of them are "live rail" issues the media and politicians won't touch.