If scanning for CP is allowed, why would it stop there? The justification is that it's harmful and illegal, but there are a lot of things you can have on your phone that are illegal. Why not scan our documents for threats? Why not scan for plans to blow up buildings? How about check our bank accounts for fraud and embezzlement? Those are all harmful and illegal. Why not check our text messages just in case we're discussing any crimes we may have committed?

I'm not even reductio ad absurdum-ing here. These are all crimes and they're all stored on your phone, thus they fall perfectly in line with the original reasoning. If committing a crime is a valid reason to remove the expectation of privacy, then there is absolutely no reason to believe that wouldn't be applied to all crimes. And there is also no reason to believe that once they have that power, they won't change the definition of "crime" to conveniently dispose of dissidents, as they are doing right now as we speak.

"The slippery slope is a fallacy" is one of the more harmful lies the west believes.