I am reviving this thread to share my experience of using the Apple Watch as primary device and thanks to the OP for starting this thread.
TL;DR it can be my primary cellular device, but with some limitations. It couldn't be my sole device.
I have been playing with the idea of using the Apple Watch as primary device for months, then I pulled the trigger for an AW S4 cellular and a pair of AirPods... guys that changed everything. I still carried the iPhone with me, until a couple of weeks ago I decided to test going solo with AW and leaving my iPhone at home.
If any of you have a fairly active lifestyle, and active doesn't mean just going to the gym or for a short run, I can see the Apple Watch being a primary device. I work 9 to 6 in an office, but when I am home I have discovered there are multiple situations when taking out my iPhone from my pocket was just inconvenient. There are multiple situations where having a computer on your wrist can help... I have a big dog that is quite demanding in terms of walks (and when he sees other dogs he just go nuts and want to play)... when you do grocery shopping and walk with lots of shopping bags, or carry anything else... when it's raining outside and one hand is busy holding up the umbrella... and if you have baby... well you understand what it means. In all these situations, AW really helps in being connected while decluttering the digital world from unnecessary stuff and the convenience of being connected without pulling out a giant slab of glass from your pocket, when your hands are already busy, is priceless. During that week when I tried the Apple Watch as my primary device, my girlfriend (she's pregnant) has been put in absolute rest by her doc, so it became even more helpful for me because I had to cook, do housekeeping stuff, and so on.
I don't think Apple Watch will ever be the one and only device we use. Each device has its own purpose. I couldn't imagine doing my job just using an Apple Watch or even an iPhone... I need a MacBook Pro. I couldn't imagine watching movies on my Apple Watch... I would need an Apple TV or an iPad... I couldn't imagine booking my next trip or buy my next pair of shirts online with an Apple Watch or shooting a movie with it.
I do think though that the Apple Watch can be a primary cellular device, especially when I am outside or driving or home busy doing stuff like cooking for example. I would always need a bigger screen, be it an iPad Mini, iPad Pro or a big iPhone, for those situations when I want to consume media or I need to produce something. But for anything else I can see the Apple Watch that could have a more prominent role in reducing drastically the usage of iPhones and in general changing the way we relate with smartphones.
I see some limitations though:
1) I live in Italy, so having Whatsapp it's a no brainer for me (even if I hate anything Facebook does)
2) If I decide to use SMS, there are very big limitations in how they are handled... I think that's because, even with LTE, the Apple Watch is a secondary cellular device. I hope they can overcome this limitation in the future.
3) I very need the notes app!!
4) One of the biggest issues is CarPlay... yes, we need Apple Watch to have CarPlay support, so I can control Music, Maps and Phone Calls from my car and leaving my iPhone at home
5) When I am abroad, I need to have my iPhone with me (not sure why)
6) You always need AirPods with you if you want privacy during phone calls
These are just 6 limitations I have found among other minor, but I see the direction Apple Watch is heading with Watch OS 6. I truly believe AW is the most exciting piece of tech of last 5 years.
If Apple allow us to use AW as a primary cellular device and pair it with the iPad, I will definitely see a future where I only use my Apple Watch when I am outside or in general when I am busy doing something (even at home) + the iPad when I am in chill out mode and I need a big screen to browse videos, read something, etc.
Last thing: I love sci fi and of course I believe in the very next future our setup will be AW+AirPods+Glasses... and a slab of glass at home for convenience.