"will need portable readers they can hand"
You could be surprised to know that exist handheld wireless POS with an embedded printer for the receipt - and they've been in use for years here. Just it looks that too many in the US think "innovation" is a one time thing, you do it once, and never update it. That's usually how civilizations ends.
Still, the "all eggs in one basket" is something that worries me - having different payment systems available ensure at least one should be available when needed. And if they don't require power, the better.