As with many things, it’s a duopoly with Mastercard that has another 30% of the market.
- Monopoly market share
- Negative impact to customers
(e.g. higher prices)
- Collusion to thwart competition
The latter two are more relevant to modern global enterprise competition. A duopoly isn't really optimally lowering prices for anything, as there's limited risk of defection and price signaling is pretty easy.