Majority of EU population. Even in US debit is more popular than credit in 18-25 age bracket.

People who don't have credit? I used a debit card at one point, though I don't anymore.

But also, they're looking at moving their credit cards to Discover as well, which would make huge waves (both in the credit card/banking world, and for their customers, who would probably find it very annoying).

Nearly every transaction account in Australia now uses a debit card as the access card, usually Visa debit. Some people will have a credit card in addition to that.

Other than merchant transactions, the CapitalOne MC card was one of the recommended cards for overseas ATM withdrawal, so the transition to a different network with almost zero international coverage has been very jarring.

I use mine at Costco for purchases over $300 (limit for tap). At least here in Canada, they only accept Mastercard, not Visa, and I don't remember the PIN for my Mastercard.

Not uncommon in Canada as far as I can tell. Lower fees for the merchant which I care about when buying locally.

People who don't enjoy debt?

Setting your incredulity aside, I'm curious why you think using a debit card would be so shocking. I effectively don't use a credit card at all: I use a debit card (or an equivalent Apple Pay representation thereof) exclusively. From my perspective, if I want something and I have the money, I'll pay for it. If I want something and I don't have the money, I won't pay for it. I don't often want things outside my budget (and I am not well-off, as a grad student), so I don't often feel any pressure to amortize the purchase over time with a credit card. And I prefer that state of affairs, because I don't want to get in the habit of using someone else's money if I can't afford to pay them back.

This isn't a value judgment on people who do use credit cards. There are plenty of reasons why using a credit card by default would be appropriate, and I'm not shocked to hear of someone who does so. But I am curious where your shock comes from, so I shared my story as a data point.