Most merchants don't work with Visa and Mastercard, they work with payment processors like Fiserv, or other middle men even further removed from the card networks, that already abstract all the different cards (including existing local debit cards) away into a unified flow.

Sure, there's a gajillion of those processors all over the world, and they somehow all work with mastercard and visa.

Diners club? Well.. "it depends". Many don't work with it.

Indian, russian, chinese, cards? Maybe in india, russia and china, but you shouldn't expect it to work "everywhere" like visa and mastercard. Same will be true for EU cards for quite a few years, especially if the system gets fragmented into many different companies using many different systems, and you'll always wonder if your german card will get accepted in Algeria like your friends' french card is.