The Apple watch will dominate all health and fitness based use cases though.
You'll still probably carry your iPhone with you, but it'll probably stay in your pocket or bag more often than not. And you're right, the Apple Watch will never replace the functions you cited.
I've been using smartphones for years, and I'm definitely someone that's very tied to e-mails and phone calls. Yet I'd rather remove the distraction. I don't need to be 100% available all day, everyday.
The Watch will let me respond to e-mails, messages, and phone calls without pulling out my phone. If it's something more involved, I can pull out my phone. Beyond that, I can wait until I get back to my desk.
The Watch + AirPods won't completely replace the iPhone, it will complement it. Just like the iPad and MacBook does. None of those verticals are particularly large for Apple, and will never touch the iPhone. They simply mop up the periphery users like me that will actually use it. Everyone else is perfectly served by the iPhone.
We are having one of those conversations about the future with current technology at current prices? Again...
(I'm somewhat resentful of journalists always interjecting the narrative in articles about the Watch that it's an underlying symptom of people being addicted to their phones. For a lot of people, being available is part of the job. The Apple Watch is the modern equivalent of a pager.)
And while minor, the geek in me smiles every time my MBP logs me in automatically using the AW.
There's a picture on this page where the Apple Watch shows "current" heart rate from "2m ago", so yes it can possibly be intermittent. Perhaps they can detect arrhythmia without continuous measurements. https://www.apple.com/newsroom/2017/09/apple-watch-series-3-...
My point was that the new OS update must track continuously or more frequently than it was originally done because it wouldn't be able to detect arrhythmia otherwise. It already tracks continuously when you're doing a workout or it senses a heart rate reading that's greater than your resting heart rate and the technology that is used on the Apple Watch to detect the rate is exactly the same as the PurePulse found on Fitbits. It's green LEDs and photodiodes. Unlike the FitBit, though, the Apple Watch also offers infrared heart rate detection, although that's only used as a backup.
I don't know where you're getting the "or it senses a heart rate reading that's greater" part. Outside of workout mode it measures intermittently when you're still according to https://support.apple.com/en-us/HT204666 "When does Apple Watch measure your heart rate".
There's more to technology than hardware. Apple Watch can't do continuous all-day heart rate like Fitbit does even though the hardware is largely the same.
Fitness-specific devices (e.g. fitbit) seem more popular than the general-purpose apple watch... at least, so far.
BTW non-screen control (eg voice control, buttons on headphones) seem logical for watches, but apple experimented with this already with the iPod shuffle, but removed it in the next version.
Compared to GPS sports watches? How do you figure?
Regarding your point, I think it's probably right. Just like compact cameras are better at taking pictures than smartphones are, a lot of people will probably settle for good enough and use a device that can do lots of things pretty well.
Garmin is selling more of their watches than ever before. This has the same useless HR, no external sensors and battery won't last through a marathon. It's an expensive gadget for people who like to think they are exercising by doing 10 repeats of 5 kg weights and 5 minutes of social media in between.