There is a much more plausible explanation, which is that they genuinely believe that more people would be saved if the security services could access messaging

There is a much more plausible explanation, which is that they genuinely believe that more people would be saved if the security services could access messaging apps and they don't think the level of potential abuse is worth worrying about. There's an even more plausible explanation, which is that both are true. They're aware that stopping encryption might save a few children, but they're also aware that it enables the mechanisms of the surveillance state that give them extraordinary power over the public. They want to stop child sex trafficking and they want to abuse people's privacy.