It is a fallacy by default. The existence and slipperiness of the slope must be justified to make it not a fallacy.

The slipperiness comes from the fact that the EU already admits that scanning of private messages didn't improve the catching and prosecution of perpetrators. Also, the biggest lobbyists for breaking E2EE argue that criminals are moving to encrypted platforms, and targeting encrypted platforms is actually the thing we need to finally put a dent in stopping the dissemination of CSAM