I understand where you're coming from because I'm not young by any means. I lived a long time without any sort of cell phone. I had a house phone that connected to the wall. I was alive when cell phones were car phones then they became portable with a carry pack, then a brick. They were expensive so I couldn't afford one. My first cell phone was a Nokia 5165. I believe they called them bar or candy bar phones. Then I went to a flip phone, and finally I broke down and bought a HTC smartphone because T9 texting was killing me when I needed to send texts for work
Just because everything worked and people lived without a particular device doesn't make it not useful. Air conditioning hasn't been around for very long in the grand scheme of things. Motor vehicles haven't been around forever.
I don't think one could say the "core function" of the iPhone is a phone. Is the core function of the Apple Watch to tell time? I never wore a watch before so I didn't need time on my wrist. My phone was working just fine as a clock/watch. If you don't want or feel the need for a smartphone there's absolutely nothing wrong with that. If you're happy using separate devices for a calculator, camera, game console, etc then it works for you. This doesn't change the fact that for many people the smartphone has replaced or at least supplemented many of those items. Most people I know use the phone function of a smartphone the least out of all the features. They are tablet computers with phone functionality.
Edit: 30's isn't old