Isn't the accessory line pretty blurred with a device like the Apple Watch? The Apple Watch is about as powerful as computers from the 2000s (if not even more recently)[1] & has it's own line of accessories [2], & has its own cellular & WiFi connections. In theory Apple could make setup not require an iPhone if they wanted & even allow wireless displays. Smartwatches really are more companion devices that are tethered to phones to speed up the setup/input process & the phone acts like a smarter virtual display.

[1] Can't find the speed of the latest one but the S5 is a 64-bit dual-core 2.4 Ghz CPU (I think the S6 is 20% faster than that?) with 1 GB of RAM & 32GB of storage. That's a pretty hefty device & will eventually consume a lot of needs for a phone. [2] https://www.apple.com/shop/watch/watch-accessories