Plenty of places don't take American Express or Discover even now, but most will usually take VISA and Mastercard.

Costco's a weird one though because they resisted credit cards for a long time until they had their Amex contract with the dual-branded Costco Amex card, but then dropped them for a dual-branded VISA card in 2016.

In most of Europe and several large Asian cities I visited, AMEX and Discover acceptance is far lower than Visa/MC (which charge lower fees to merchants).

AMEX/Discover is virtually unknown in most of Europe.

Outside of US-defaultism, why in the world would anyone expect it to be accepted here?

Diner's Club is equivalent to Discover card and is at least somewhat present in European countries. I don't know official numbers but I've seen it often enough and have used it once or twice.

“Amex is virtually unknown in most of Europe” doesn’t match my experience. Most major European economies have AmEx and AmEx issues many local cards to them

Doesn't Discover have agreements with UnionPay and JCB?

Fun fact: AMEX has been involved in international payments in one way or another for more than a 100 years and the AMEX card was the first international charge card, launched the same year as the original BofA Visa Card.