The precedent of the Microsoft IE anti-trust case basically refutes your core arguments. It's not about Apple being a literal monopoly, it's about Apple holding a dominate position from which they make abusive anti competitive and anti consumer decisions which are propagated to the rest of the market. App developers are going to look at iOS market's dominate revenue stream and decided that if they are to maximize their potential profitability as per their likely fiduciary duty they will need to follow Apple's whims.