A monopoly has structural implications which could disadvantage both customers and developers in ways that go beyond paying x% Apple Tax on each sale.
For example - opportunity costs for both developers and customers, lock-in costs, costs to competitors who might have been able to operate a related market, specific losses for specific developers based on arbitrary, capricious, or self-serving app store exclusions, and so on.
None of this has been proven yet, all of it is debatable, and it could all be included in potential arguments.
This is bad news for Apple - not just because any judgement could be worth tens of billions, but because of potential bad PR and brand damage, which may happen however the final judgement goes. And also because of the distraction from more constructive goals.