Well they'll go for the service providers, of course.
Signal would effectively have to leave the EU market and block any EU users to stay out of hot water.
The list of privacy-respecting chat apps would become real short real fast, and good luck getting everyone in your life to use one.
Yeah, I have my own matrix instance, but unless I want to cut off 90% of the people I want to have in my life, I can't not bridge it to at least telegram and whatsapp.
It doesn't matter that this is unenforceable, or that alternatives exists. That simply means that those of us who care will still be able to keep some of our communications secure. But this legal change will still make it impossible to keep all of our communications private. That's already the case, and this will make it orders of magnitude worse.
Unenforceable? On an individual level, yes. On a societal level? No. This absolutely can and will enable the monitoring of 99.99% of actual chat activity.