I wonder if Apple truly thought that "apps" weren't needed, and gave in to the public outcry to create an SDK, or if they had this plan all along and just withheld it at the beginning to generate excitement and rumor mill traffic.
Certainly the apps are what carried the iPhone (and modern smartphones) to their current levels of success. An iPhone without apps was (and still would be today) pretty boring.
If Apple truly thought that app SDKs were unnecessary then that would have been a huge mistake -- one that they corrected, thankfully, and in doing so reaped billions of dollars in return.
I'd guess they planned to introduce 3rd party apps all along. But getting the phone & iPhone OS done on schedule was enough of an engineering challenge, without adding 3rd party apps into the mix. It was almost an instant reaction for a lot of people: "Oooh! Can I make apps for that?". I can't imagine Apple could have missed that. IIRC, Apple were ridiculing the notion of them releasing a phone... until the first rumours leaked out. It probably was the same with apps, there's little point in announcing them too far in advance.
IMO, the direct revenue from apps is probably a lesser benefit than the buzz it generates around the iOS devices; and the 'lock in' factor. I love the look of the latest Samsung Galaxy, but it would be hard to leave the many dozens of purchased iOS apps behind.