And the reason for the huge backlack, is that this stop gap will actually make it easier for them to request more afterwards, because the infrastructure, the proof of concept, will already be there and running. And it will cross to other providers: "see Apple does it, so clearly it's Signal that's being protective of criminals, we should impose them to do the same thing Apple did with no issue".
I simply disagree with the notion that I should be controlled and monitored by a third party just because someone else might do something evil.
We should always remember that power corrupts and definitions of evil change almost on a whim.
I could be missing something, but I did a quick search and all I see is news about them scrapping their once-encrypted backups at the request of the FBI.
The arguments about slippery slopes and potential surveillance weren't as interesting to me as the opposing argument: that a very high level of privacy (not even an absolute level) carries consequences for a specific segment of society by the intrinsic nature of what is kept private, and in the name of protecting that segment of society, the tradeoff is not worth it.
There is also the idea that data on a hard drive can be as damaging to human livelihood as physical contraband, to the point that the vast majority of the world's legal systems, not just those of the U.S., have decided that the data should not exist under any circumstances. CSAM is one of the few classes of digital data that compels the creation of scanning systems for such data on a scale that isn't driven by political ideology, propaganda or similar. It's difficult to imagine how Apple would be obliged and driven enough to implement such a system out in the open and in the name of the public good if the publicly announced reasoning was to scan any other class of data (assuming that Apple can be trusted, at least).
I don't know why we accept the same things when we're talking about digital privacy.