Seeing Apple's response here gives me a pretty good idea as to how they will likely respond to the side loading lawsuit with the EU.
Attempt to drag out the case in court for as long as possible, while buying time to work out a solution which does the bare minimum to allow side loading, while also making it as inconvenient for all parties involved, to the point where it becomes unfeasible for all but the most hardcore of users.
For example, I imagine Apple might decide that sideloading apps may void your AppleCare warranty one day, or maybe not be eligible for certain key iOS features like iCloud.
I guess the question is not so much about whether Apple will allow sideloading, but in what shape and form?