My parents live in Germany and card payments are only starting to permeate.
They now seem to have the V-Pay and Maestro debit schemes on cards in favour of their domestic girocard, but VISA and MasterCard are still not generally well accepted outside major retailers and big cities. It’s for that reason that issuers like N26 and Revolut offer Maestro cards in selected markets like Germany and The Netherlands - if they didn’t, VISA and MasterCards holders wouldn’t be able to use their cards at all. So far from retailers adapting, it is the card issuers that are adapting to meet the needs of the local markets in which they serve.
I’ve had a number of conversations with my parents, relatives, and friends on the subject of card acceptance in Germany, and they always give me the same two answers: (1) the general population do not trust banks, and (2) cards just increase the cost of things. This isn’t just a generational thing either - it’s the same from twentysomethings upwards.
I believe that the EU cap on fees is a significant reason that more retailers are starting to accept international cards in Germany, but card payments are still miles behind the levels found here. Cash is very much king. And to give you an idea of how antiquated things are (from my own perspective), my parents regularly order stuff online yet pay cash-on-delivery.
As Germans still don’t trust banks, I wouldn’t give them any reason to distrust further by removing the fee cap. If the EU did that, Germany would just pull up the drawbridge and return to their own domestic girocard product. It’s a cultural thing. And culture doesn’t change quickly.