One of the main arguments for the relevance of COBOL is that it handled „95% of all Creditcard-Swipes“ or „95% of all In-Person Transactions“ or „95% of all ATM Swipes“.

This number, like many other numbers, comes from the Gartner survey in 1997. So this number, again, only says that 95%
1. of mainframe owners
2. who handle creditcard swipes
utilize their mainframes for handling creditcard swipes.

Given the small number of 421 participants in the survey and the divisibility by 5, I suspect that we are talking about 100/5=20 banks – and 19 of them using their mainframes for processing creditcard swipes. Even if it were 399 of 420 banks – scaling such a small number up to the entire world market is unsound to say the least.

Today, there are only about 10,000 mainframes left in the world, but there are 25,000 banks in the world. So even assuming that all mainframes are owned by one bank each – and assuming that these banks handle 100% of their creditcard swipes with these mainframes, maximum likelyhood still says that COBOL handles no more than 40% of all creditcard-swipes. And since the assumptions were quite unrealistic, the actual number is probably even much less.