Certainly doesn't help that it's the current 30% but it being 15% won't change the fundamental issues being argued - whether Apple is abusing its market power and shutting competitors out of or subjecting them to unfair practices in its iOS ecosystem. E.g. unfairly favoring its own Music app.
I think it's a fairly straightforward case similar to Microsoft's antitrust case for bundling the Internet Explorer in its Windows OS a couple of decades ago. The argument then was that Microsoft favored its own browser and crowded out viable competitors like Netscape. If anything, the case is more obvious here since you can't even install any other music streaming alternatives on your iPhone if it's not in the App Store - in Windows you could download and install another browser (even if most users didn't).