“Does the iPhone obviate a standalone digital camera or the need for a landline phone?”
The answer is (talking broadly) no and no. Most people are not photographers, they only take photos (two very different things), and most don’t need the extra cost for a landline phone, that is so “stone age” anyway. If you have kids beyond 10 years of age they have mobile phones themselves.
“it replaces the standalone digital and video camera in many instances.”
“rendering a standalone MP3 player or GPS device unnecessary.”
We are still some way off when it comes to replacing the wallet and ID card.