Apple is obviously already scanning for CSAM for whatever non-E2E content hits their servers. With this, they offload some of the cost of computation to the end nodes but that can’t be the only reason for pushing this tech, so it’s only natural to ask why and look for avenues of exploitation. The Trojan horse/foot-in-the-door hypothesis is basically Occam’s razor.
How will you know? And actually, how will Apple know that's all their doing? They won't because all they can see is a hash, so they don't actually know if they're checking for anti-winnie the pooh memes, or CP. And there is no public auditing, there is no auditing by your senators, there is no public disclosure of the hashes they are checking, and no public disclosure of the exact algorithm used. And even if they tried to do the latter two, you'd have no way of verifying it.
You said earlier upthread, that HN is ignoring the tradeoffs. I find that distasteful, people have considered the tradeoffs of this and they don't like them, but that doesn't mean they need to preface every single statement about the thing with "well actually i've considered the tradeoffs, and I've found them to be blah, and..." That would quickly get tiresome which is why people don't do it.
I'll make a different, similar generalization. Like most people pushing this, you are pretending to care about the "tradeoffs", but you don't. You don't care about the downsides, you just say you do, and then blithely ignore them when they are brought up. What's the golden rule again?