If you're going to define "monopoly" and "platform" than the word monopoly loses all meaning. If I create a business making 3rd-party wire harnesses for VW Passat and VW decides to change to a different harness and that change would kill my business, should there be some regulatory body that can force VW to stick to its old tech?
iOS or iOS-based devices are not a market. Smartphones or tablet computers or personal computers are markets.
Consider that the government concern with Microsoft in the 90s wasn't its monopoly position in "Windows," which is tautological, but PC operating systems broadly, which includes Linux and Macs.
The smartphone and tablet computer markets are among the most competitive in the country right now. By definition this means no one has a monopoly.
"The relevant market consists of a 'catalogue' of goods and/or services which are considered substitutes by the customer. Such a catalogue is considered 'worth monopolising' if should only one single supplier provided it, that supplier could profitably increase its price without its customers turning away and choosing other goods and services from other suppliers."
So the question is this: if Apple were to increase its share of revenue by 5% to 10%, would developers move to Android, BlackBerry, or WP7? Absolutely. Even at the current level, there is plenty of cross-over between Android and iOS applications. In antitrust terms, the AppStore is not a market.
Telecommunications (in the USA) used publicly-owned airwaves and in too-many cases have de facto or de jure monopolies. Neither applies to Apple.