1. Not everyone owns a smartphone.
2. You are relying on an external device that may not always be available (lost/stolen/broken/in a repair shop = no Switch online for you).
3. Phone runs out of battery mid-game, you're probably booted out of your online session.
4. Many people install iOS/Android public betas, the app could potentially be broken on these beta OS's meaning these users cannot play online for months...
5. They are putting themselves in the hands of others - Apple's App Store goes on lockdown in like the last 2-3 weeks of December, and that's when there's most likely gonna be a problem if Switch (somehow) gets massive holiday sales. So they could be in a situation where they need to release an emergency app update and all the App Store employees are on Christmas vacation or whatever.
6. This ties up your phone. If you answer a phone call, it'll almost certainly end your online session (or at least the voice chat aspect).
Then there's the logistics of this...
7. You're not gonna be able to hold a cell phone to your ear while both hands are clutching a controller. So this leaves two options, headset or speakerphone.
8. Headset - the lightning earbuds included with the iPhone 7 block the charging port, so you are going to run into issue #3 (battery dying mid-game). Unless you buy a bluetooth headset, but then you run into more issues (hassles pairing / re-pairing the BT connection, another device to purchase, another device you have to monitor the battery levels of, etc.)
9. Speakerphone - is what most people will probably use and that sounds crappy. Since the phone is most likely going to be sitting on a coffee table or the other side of your couch, voices are going to be garbled as fuck, you're gonna hear the game noise on top of that, etc.
This is one of the worst ideas I've ever heard, not a single thing about this setup sounds the least bit appealing or convenient.