If they can get cellular in there and keep a full day battery life with limited data use and half an hour of talk time then I could pretty much give up on my phone and go with the watch and an iPad. The iPad has a usable interface, and the watch/airpods are nice and portable. I'm not someone who uses my phone all day, so I don't need those radios on very much during the day.
I would imagine that would be a lot to ask out of the first cellular model. But I guarantee that Apple will set it up so that when your watch is in range of the iPhone, it will turn off the cellular radio, and back on again when you separate yourself from it. I would also expect the user to have the option to set what conditions that the radio be used, just as you can on the iPhone when roaming, etc.
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See no point of this, I will have my phone with me all the time. I can't imagine people not leaving their iPhones behind for any reason.
Enjoy that reality. I'll be living untethered to my phone as needed.