I think such a system could be made convenient. And if the alternative is not legally binding, like, if they want to use magic numbers and paper and you give them the number and sign with ink and they give you money then come to collect, and all you have to say is "show me the digital signature", and they produce it or by law they have no claim, it'll get figured out and convenient.

Europeans all own a little gizmo that takes a plastic card, which they use to authorize bank exchanges.

It's not a big deal, and the proposal you're dismissing has an identical user experience.

With 99% of pockets containing an NFC chipped phone these days, it could be even easier than that.