iOS is no longer the leading OS in new mobile phones, but sales and profits continue to increase, and with Apple's ecosystem in place, people already invested in that ecosystem are likely to stay with it.
Sure, but...
1) If someone is invested in the Appel's ecosystem, that person already has the iPhone 3 family or the iPhone 4. Why would someone who has the iPhone 4 (or the iPhone 3GS, but didn't want to upgrade to the iPhone 4) upgrade to the iPhone 4S? Slightly increased speed and slightly better camera are likely not going to make that many people change their current iPhone.
2) If someone does not already have an iPhone, it could be because that someone does not like the current iPhones, or due to their cost (among other reasons). The iPhone 4S (if it's the only iPhone being released) is likely not going to be any cheaper than the current iPhone 4, so those who can't afford an iPhone 4 won't buy the iPhone 4S; and those who don't like the iPhone 4 are going to see basically the same thing when they look at the iPhone 4S.