It seems to me the most likely outcome, if the DOJ is successful, is that at some point in the future Apple users will be offered a paid service, simulating today's "walled garden". Of course it will be a compromise, but they're probably already working on it. If Apple's marketing team has the nerve for it, it may even be called Walled Garden™. Only Apple devices will be supported, of course.

Everybody who values the benefits of Apple's ecosystem today will pay for it. And obviously it's not only the customers who will pay, developers will be offered the option of supporting Walled Garden™ features when they release their apps. Most likely, the additional terms will require that they pay exactly what they pay today. And since the customers will be flocking there for the added security, curated App Store content, and enhanced, worry-free ecosystem benefits, all the developers who are whinging about it now will be forced to sign up to stay in business.

End result: nothing changes, but everybody pays more. Nice one.