I'm 22 years old, a college graduate, and still using the same phone I got during my freshman year of high school. It has a flip-out keyboard, only calls on speaker phone, the inside screen is nearly impossible to read, and until I lodged something under the battery to hold it in place, it would shut off almost every time I put it in my pocket. Middle schoolers in the youth group I lead make fun of me for it. So why not upgrade to a new phone?
First off, it isn't a matter of cost. At least financially. It no longer is much more expensive to find a used smartphone online and to get a data plan than what it is to use a dumbphone. For me, it's probably what you would guess: I would rather not be too tied to my phone, distracted by constant notifications (Yes, I know that they can be turned off, but who actually turns their notifications off?) I prefer to be able to not even have the option of checking those things. I don't need it. That way it is out of mind, as I don't even see it as a possibility.
I find that there really isn't that much I miss out on while having this phone. In fact, I'd argue that my quality of life is better while having my current phone than it would be with a smartphone. There are obviously some things I wouldn't mind having at times for sake of convenience, but honestly those times are extremely rare, and never having owned a smartphone, I rarely even think about those things. I find that I am able to be much more present, and to more fully engage with the world. I don't have to worry how many people are viewing my Snapchats, or to monitor likes on Facebook posts. As we all know, those things can really drain more life out of us than they give most of the time.
It's interesting to me how quickly people comment on my phone now that smartphones are basically owned by everyone, especially in my generation. The reception is generally positive, but people tend to wonder why I choose not to upgrade, and I basically just tell them what I mentioned above: that I can easily live without it, and prefer to have less distraction. Many of them often tell me that they are thinking about switching back to a dumbphone (which I doubt anyone actually does), or tell me they miss the days when they had a dumbphone, and it makes me really appreciate the luxury I have been able to have of never having owned a smartphone. How long this will last, I cannot say for sure. What I can say is that I plan on holding out for as long as I can.