I want to experience being a self-sufficient adult in a world without smartphones, and I don't care if that's a boring answer.
Everything the smartphone has touched has turned to garbage and nearly everything I dislike about the modern world can be tied into smartphone usage in some way. They're unavoidable. Even if you don't carry one yourself you still have to deal with living in a world where everyone else does. It's still possible to get by without owning one, but every year it seems to become less convenient. As much as I hate them, I own one myself and can't blame anyone else for doing so. They've made themselves necessary. Modern restaurants, banks, schools, parking garages, public transportation, etc. are all designed around the assumption that you have a smartphone. Social interaction, even if you're a grown-up, revolves around the smartphone. You need a messaging app to communicate with anyone. You're expected to have a phone camera on hand to document social events to prove that they happened. You can't even go on a date anymore without submitting a photo gallery of your everyday life beforehand, like you're listing yourself for sale on eBay.
I'm so sick of it all. It's not enough to get rid of my own smartphone, I want to live in a world where smartphones aren't necessary at all and where I don't have to deal with their side effects. That's the one thing I miss the most when I think about life in previous generations.