I tried running with a smartphone for a while. I was constantly afraid I would drop it. I tried armbands, pockets, sometimes I just carried a stopwatch and logged all my data in a spreadsheet like some kind of neanderthal.
Eventually though I bought a watch. It can tell me how I'm doing on my split times and play music into a cheap bluetooth headband. It's super lightweight. It can even detect different types of weightlifting, and tell me I just did a set of five deadlifts, or squats. It somehow knows the difference. It doesn't seem to understand sprinting at all though and has all sorts of weird bugs because I assume it has a small userbase.
I'm not sure any of my apps aside from activity tracking and music make any sense on a smaller device, so that is kind of weird. It's like I strap a tiny phone to my wrist in the mornings, then I swap it out for a larger copy an hour later.
Maybe one day phones will be like other accessories and we'll have eight of them and complain both about having too many and how we want more of them. And each night we'll debate internally about which one to wear to a certain function, and judge each other mercilessly for being over- or under-phoned.