Q: All indications appear that the iPhone is closed, we’d love to develop apps…

This is an important tradeoff between security and openness. We want both. We’re working through a way… we’ll find a way to let 3rd parties write apps and still preserve security on the iPhone. But until we find that way we can’t compromise the security of the phone. I’ve used 3rd party apps… the more you add, the more your phone crashes. No one’s perfect, and we’d sure like our phone not to crash once a day. If you can just be a little more patient with us I think everyone can get what they want.

https://www.engadget.com/2007-05-30-steve-jobs-live-from-d-2007.html

I think apps on the iPhone will be a very big new market.