The Android app store certainly exists, and is doing quite well. The web is free for all platforms. Apple don't, by any realistic measure, have a monopoly on application sales for smartphones. They just couldn't - with only 25% of the smartphone market, and an app store that targets only that 25%, how can they have a monopoly on smartphone app publication? What they might have is a monopoly on the wealthy gadget consumer with large amounts of disposable income to spend on apps. I'm not, however, that Apple's clever demographic targeting, coupled with the inability of their competitors to eat into this market via fair competition, is reason to force them to change their business model.
So I don't think we can say "they just couldn't" — in fact, they just did.
And I might be skeptical about the numbers (Gartner isn't infallible), but every story I've heard from cross-platform mobile dev houses agrees with this — you'll get almost 100 times the sales on iPhone that you will on other platforms. I've never heard a contradictory story either, not even from Google trying to puff itself up. That's pretty much a monopoly.