I like your thoughts here. But I wouldn't be too concerned about not having the possibility of 'all day battery'. The Apple Watch is dependent on how you use it. But for the sake of having LTE and the freedom of not having your iPhone tethered to you when you go somewhere briefly, I think that alone would be a huge advantage if you can appreciate that. As you mentioned, $10 a month to your carrier, is only justifiable if it's convenient enough for you.
But being I use an iPhone 7 Plus primarily, I also would rather not have to have that with me all the time, being its such a large phone.