The vast, vast majority of 'regular' iPhone users have never, and will never hear of the apps described in this post. For them, the default apps are just fine.
That's not to disparage the choice, just to recognise that regular users don't care about this kind of thing one bit.
That would imply the App Store is completely useless and not a huge factor for the continuous success of iOS devices. I think you can see how wrong you are.
The thing is, not everyone will customize all apps, but everyone will customize at least one app, be it a better todo list, a better IM, a better picture-taking app, or a better recipe database. Which is why the appstore is so popular.
EDIT: clearly the "regular users" I know are different from everybody else. The North of England must be a hive of phone-geeks living in some sort of bubble.
Everyone will customise their experience of the device, by adding what they like from the store.
My point is that, more often than not, these users won't replace core apps like Mail or Calendar - these apps work just fine for most people and there's no incentive to change.
There is no incentive... until there is. The default camera app doesn't zoom. The default music player won't create playlists (edit -- sorry, it does; what I meant is that there is no queuing to the now-playing like in old iPods). The default todo app won't do cloud sync (well, it does now, but it wasn't the case before). And so on -- as soon as your usage of a particular app increases over a certain threshold, you'll encounter a corner case and head for the appstore to get a better app. So a photographer will have five picture-related apps and never touch the default one, while using the default mail client; a lifehacker will have fifteen hyper-specialized ToDo apps and never touch the default one, while using the default camera app.
I don't mean to be nitpicky, but you can zoom in the default camera app with the pinching gesture. And the only todo app for the iPhone has had iCloud support from day one, since it was first released with iOS5.
The vast, vast majority of 'regular' iPhone users have never, and will never hear of the apps described in this post. For them, the default apps are just fine.
That's not to disparage the choice, just to recognise that regular users don't care about this kind of thing one bit.