Yeah but messaging apps are really only useful if there are lots of people on them to message.
So in the real world a relatively small number of providers, WhatsApp, Signal etc, are in a position where all your friends are going to be on them. And those are the ones likely to be named and told they need to implement image scanning/review.
Messaging protocols are useful even if everyone is not on the same app. In the past I was chatting with my google using friend via some third party jabber server where I had an account. It was useful and didn't require us to be "in the same app". We both were using both different apps and different server providers.
It doesn't have to be though. There is countless benefits to decentralized and federated platforms, even for your average Joe.
It's just that all the Google, Microsoft, and Meta platforms had shiny new features and we all switched. There is no real reason we can't go back, sure the network effect is hard to overcome but the technological problem is moderately simple to solve (we did it in the past!)
> As part of changing laws in Europe, Meta now offers the option for you to chat with others using third-party messaging apps that have integrated with WhatsApp and that you choose to turn on. - Whatsapp Help Center
Currently support is a bit shit, given it's relatively new. Give it 3-5 years and I'm sure this will look very different.
But actual protocols are so last century. You might have to think ahead for fifteen minutes because the design has to be staaaa-a-ble. It's haa-a-ard! And you can't sell out to somebody who'll change it and have an exit event.
So in the real world a relatively small number of providers, WhatsApp, Signal etc, are in a position where all your friends are going to be on them. And those are the ones likely to be named and told they need to implement image scanning/review.