I think it's going to be extremely hard to prove that either customers or developers suffer because of the monopoly.
The pricing point is clearly wrong. Most apps are priced ridiculously low for the value they offer. I don't see that Apple can be blamed for that.
The lock-in part is also wrong. Developers are free to develop apps for Android (or Windows, if they really want to). There are even cross-platform all-in-one dev kits. (Admittedly they're not great, but it's possible to use them to do some large percentage of the dev work on a cross-platform app and then fill in the native details later.)
There are certainly elements that suck. Discovery on the app store is awful, and continues to be awful. Too many apps are still abandonware, even after last year's clean out.
Ironically developers might benefit more from better curation. A smaller number of really solid, bug-free apps would be better than the current mess of "I have no idea if this is going to crash or not" free for all.
But that's not really an anti-trust issue.
I'll be very surprised if this case gets far. It's clearly on thinner ice than the Bell, IBM, and MS antitrust cases.
If anything there's a better case to be made for hauling Google in for its near-monopoly on ad sales and search, and its habit of screwing over site owners who rely on ad revenue.