In front of me was a couple in the beginning of their 20s and none of them had a credit card or even debit card on them, because they had flown this flight a lot of times and they simply brought cash.
Well... their bag was slightly oversized and of course Ryainair wanted them to pay for it. So far so good, but Ryanair ONLY accepts credit cards at this point to pay for your extra luggage, so the flight of the couple was just about to bekome much more stressfull (remember this is 15 minutes before actually leaving with the plane and you literally have nowhere to do it in cash).
If I had not seen that exchange and traded cash for putting my card down, that couple would have had one sad vacation of two weeks.
All in all, airports are not equipped for this influx of "i will pay right now", because simply they are understaffed as is.