this kinda serves of a proof of concept for just how minimal we can get with a smartphone while retaining most of the "smart". I might even try this for a bit...
I think the parent comment's point is good -- if Apple are watching (pun not intended): you could make a truly tiny phone out of watchOS, please do it.
Hard to say without phones available that cater towards other needs. I'm waiting for one that brags about not having access to most functionality outside of phone, gps, sms, and camera.