The slippery slope argument is the only useful argument here.
The fundamental issue with their PSI/CSAM system is that they already were scanning iCloud content [1] and that they're seemingly not removing the ability to do that. If the PSI/CSAM system had been announced along side E2E encryption for iCloud backups, it would be clear that they were attempting to act in their users best interests.
But it wasn't, so as it stands there's no obvious user benefit. It then becomes a question of trust. Apple are clearly willing to add functionality at the request of governments, on device. By doing this they lose user trust (in my opinion).
> In the long run, we solve this, at least in liberal democracies, by voting people into office who understand technology, understand the value of personal encryption
But this is clearly not the way it happens in practice. At least in part, we vote with our wallets and give money to companies willing to push back on governmental over-reach. Until now, Apple was one such company [2].
I realize that Apple likely don't "care" about privacy (it's a company, not a individual human). But in a purely cynical sense, positioning themselves as caring about privacy, and pushing back against governmental over-reach on users behalf was useful. And while it's "just marketing" it benefits users.
By implementing this functionality, they've lost this "marketing benefit". Users can't buy devices believing they're supporting a company willing to defend their privacy.
[1] https://nakedsecurity.sophos.com/2020/01/09/apples-scanning-...