I used a Nokia 3310 for 12 years, starting in high school. When it finally broke (and I was a 30-ish young dad), there were always several "old phones" abandoned by family members or close relatives, so I've been using those to their end ever since. The current one is a Nokia 2600, which I genuinely dislike (tiny buttons, overengineered menu system), but the miserable thing stubbornly works, so what can I do?
IIRC, the idea of constantly reading e-mail or browsing the web from some tiny phone screen made me really uneasy even as a young adult. I surely liked computers, but that, I felt, would be too much. Would I really need anything else except SMS and direct calls from a phone, I thought? Probably not.
I loved talking to telecom salespeople when I had that 3310. "We have this new offer, so many megabytes of Internet, and... what's your current phone, by the way?" they would ask.
"A 3310," I replied.
"...Well, that's surely a classic, sir!" they would reply after a few seconds of silence.
I successfully worked as a journalist for years, whilst living in the dumb phone world. I don't remember having any problems. Neither have I ever gotten hopelessly lost on car trips with only a paper map, or hand-written directions, etc. During the last 3-4 years, though, I do sometimes feel like getting in the way of others, or of life: more and more relevant services seem to assume that the user has a smartphone. But I guess that's life.
I do like the idea of something as barebones as the Hellaphone, Inferno OS on Android phones [1, 2]. But the feature and UI bloat of current casual smartphones still makes me uneasy about getting one (I'll do it if I really have to, though). I occasionally also think about going landline- or even email-only, thus ditching mobile phones and constant availability altogether, but this would probably cause too much mess for our family.
But, re: young urban professionals ditching their smartphones after years of usage: dude, I knew in 2004 that this day will come.