I used Signal as an example.

People will use what is most convenient. If tomorrow Signal leaves the EU, WhatsApp will happily take its place and will happily enforce the scanning and everyone will just have to fall in line.

What good is it if you are the only one of your family who has the only "uncompromised" app on your phone? How will you talk to them? Any message you send will be scanned on the other end.

That also applies if you have friends overseas. Your friend from Japan/US will be compromised as well.