> Most adults will be using credit cards

This really depends on the country. In Germany, most day-to-day payments happen with cash or over the Girocard network. I got by for most of my adult life without a credit card. The only thing that's kind of tricky is booking a hotel without one (international hotel chains have standardized their registration systems around credit cards). I only got a credit card last year because it's free with my current checking account.