Signal Foundation has already said they would leave the EU if Chat Control goes ahead. https://mastodon.world/@Mer__edith/112535616774247450
Signal isn't big enough to play that game. They'll just be blocked from the app store for EU users and their user base in the EU will drop to near zero within a year.
They are not somehow bluffing or threatening this simply to try to change the law. It's a principled stance that they simply cannot provide E2E encrypted chat under such conditions. So either they break their protocol in which case their claimed offer would be a lie, or they leave.
Seems like the only choice they have, really. Also, by "leave EU" I'm pretty sure they mean not offer their app in the EU, so yes I think they expect their EU user base to be zero in this scenario.