Alternative to what exactly? Where is detailed methodology of planning CSAM? Where are trusted data that shows how many real abusers are on platforms like those mentioned to be monitored? Where are warrants that those data collection won't be abused by officials?
There will always be other platforms that provide secure communication for sharing illegal files. Massive collection of child porn sounds perfect for potential consumers.
Also, most of child abuser's are among people that are not strangers to victims, so they don't even need IM for communication.
Snowden's "turnkey tyranny"-argument is okay but requires specific knowledge and imagines things that haven't happened which therefor is open to interpretation.
I think the only real way to prevent something like this is to provide an alternative but that is also unlikely since it implies responsibility which the tech industry doesn't like.