I personally don’t think that’s really feasible unless they provide smartphones for every citizen themselves, and even then people like my grandparents would basically not be able to live given that they only barely know how to message me (and even that they do wrong sometimes, so…). They can certainly make it difficult without a smartphone, but they likely can’t completely eliminate physical IDs until those issues are gone.

EDIT: also, if that does become an issue at some point, you can keep your smartphone off unless necessary and don’t use a simcard. When on keep it in airplane mode. If you need a connection to use digital ID, do it briefly over wifi (since presumably most places you’d need digital ID would have wifi) and then turn it back off. While not perfect, this would probably be good enough for most people.