Not so, they can go to Android (and there have been other competitors that few people showed any interest in).
You could say the same thing about a dominant supermarket - as a seller if you want access to their customers sell your goods there, or hope those customers will go somewhere else for your goods. As a customer, either buy what's on offer there or go to another supermarket.
Realistically though you'll have to download other stores, for the same reason you can't really get by with only AppStore apps on a Mac. Once they have the option of selling through their own stores without the restrictions Apple imposes (such as the ATT rules which triggered this whole argument) some of the big players like Google, Facebook (obviously), Microsoft, etc will, I predict, remove their products from the App Store and sell them exclusively through less well regulated stores. Once they have opened these poorly regulated stores and ensured they have large built in customer-bases no doubt a bunch of other developers will move their own apps there - why would they comply with Apple's customer-friendly regulations when they don't have to?