Of course. If Apple stopped supporting app installation from other sources than App Store on Mac, I am sure lots of customers would leave the platform.

Developing and installing apps is the main feature of a computer. Apple emerged from this, from BSD and UNIX background, they are still using mostly Linux servers, yet they are taking this away from their users and force them into their "secure" stores which they control fully.

EU should fight this and if Apple doesn't want to complain, they should leave EU market. There will also be less electronic waste in the end...