The iPhone could be a Thread border router, but it probably shouldn’t be
The iPhone could control Thread smart home devices faster
Thread could replace Bluetooth as a way to connect peripherals like the Apple Watch to your iPhone
Thread would be fine for most smartwatch uses but not for audio streaming
Thread might just be a bonus radio
If you want to support a lot of devices in a dense apartment building with wifi you pretty much want everybody to have MU-MIMO capable access points and IoT gear. Otherwise you’re hogging spectrum.
(It’s not like other protocols aren’t also affected, but at least they don’t compete with people’s internet traffic)