think of terrorists. Suppose your country has banned/restricted selling guns and explosives. Do you think this will stop terrorists? No, it'll stop normal citizens to not do any of this stuff... but they will not be affected since they are not terrorists.
Terrorists on the other hand will find illegal ways to overcome the restrictions. It's the same with encryption - (open-source) tools with e2ee are already broadly available, sideloading is available - nothing will stop terrorists/other criminals to just install that and continue doing what they want.
On the other hand this chat control opens a huge area of opportunities for govts to spy on citizens or maybe journalists/other politicians that they dislike. And this is if we assume the system doesn't have bugs that would allow third parties/hackers to break it and get all the info by themselves, or bugs that can trigger a false-positive event.
That's why it's a bad idea. Criminals will find ways to overcome the limit, govs will get new tools for suppression (even if current govs are 'good', what if the next govt is some ultra conservative or radical nationalist, do you think they'll not use these new tools?) and normal ppl are basically left without any privacy