"Most can’t afford to write two separate interfaces. (It’s a terrible idea anyway.)"
Why? If the newcomers can afford to write an interface for iOS7, the established players also can. If there is money in it for newcomers, there is money in it for established players too. This article assumes that established players are dumb. They will estimate how users adopt iOS7 over time, and act accordingly.
I think it assumes that established players are conservative, not dumb. They will do the least effort necessary to make their app look good in iOS 7, at least at first.
Why? If the newcomers can afford to write an interface for iOS7, the established players also can. If there is money in it for newcomers, there is money in it for established players too. This article assumes that established players are dumb. They will estimate how users adopt iOS7 over time, and act accordingly.