It's just retarded, there is no need to speculate or explain anything, the very concept is extremely stupid on a fundamental level. Should every person on earth who wants to use crypto in person also have a phone? Do you know how long the battery lasts for modern phones?
we are talking not about "fundamental" future. but about tomorrow things. The main idea is to make some simply and available technology tomorrow. That is why everybody is looking around himself in order to pick up something which already exists and there is no need to spend a lot of resources in order to implement the conception
look:
smartphones
smartcards
NFC tags
SD cards
are already exist!
It's only the question of software and in the nearest future (maybe months) we will have offline transaction technology.
Of course in some distance future we will have more advanced technologies. I think we will fly instead of driving cars in the future! )) and so what....? we shouldn't think how to develop cars?