The fact that banking wasn't a cabal led to the creation of Visa & MC, they provide the function of operating a deliberately independent interchange so competitive banks can work together without having to talk. The fact they havent been replaced isnt due to a lack of "normal competition", its just a system that has built in network effects (but is sticky unlike most social networks).
Dont get me wrong - banks love cabals! Thats why domestic switches (led by central banks) are replacing the scheme networks in most countries for domestic transactions with the domestic switches being linked for international transactions, pushing Visa & MC out.
>> 2.5% of a transaction considerably too much, were there efficiency, it would be less than 0.5%.
I think your wrath is misdirected - that 2.5% may be stated in a Visa/MC press release, but it doesn't actually come from Visa or MC! The banks set interchange through the schemes. You add in 5000 different schemes thats not gonna change the fact that your bank is going to try to get the best bang for their buck if they "acquire" merchants or "issue" cards.
The porn thing is linked too, banking is a heavily regulated space, easy to put pressure on. Acquiring banks get in trouble easily, they rely on schemes to blacklist anything which could land them in hot water. Even if there was no MC/VISA banks would still be paying random service providers to operate blacklists. The only difference would be that it would be cheaper. And typically thats not a good thing.
The evidence that they are an oligarchy lies in their power to set prices. Which points right to the definition of what a monopoly is.
If such systems were truly competitive, and we had say, 5 completely different systems that were truly competitive, the price would not be a total of 2.5%. The price would be set by the market, not the providers of the service and it would be much, much lower than 2.5%.