For me dumb phone is no use, as I don't use cellular voice for anything (I've talk with my friends and parents on daily basis, but it is always Telegram/Skype voice call, not cellular voice), and only usage of SMS for me is force-feeded 2FA for banks and other services which don't know better (unfortunately)...
Who will I call, we all are all around the Globe? Why I will text?.. Ok, week ago I had birthday party and there were a fair amount of guests, so some of my friends now in same location as me. Do they have local phone numbers? I bet, they have, because without it it is impossible to open bank account or rent a flat. Do I know these numbers? No. Because for what reason, we all communicate in messengers for years. These accounts are fixed identities for us, and phone numbers are for governments and banks.
I could simply turn off my smartphone with same effect as have dumb phone :-)
Oh, maybe, I need to call ambulance once a year...
Edit:
Idea of phone which is technically LTE Modem with all chat/voice protocols under the unified phone-like interface, plus ability to run some offline GPS, like Organic Maps or OsmAnd, looks good, but politically impossible due to close nature of most chat/voice apps.