If someone owned all the highways, building an alternative would be difficult as well.
Visa and Mastercard do not own the internet cables or the wavelengths. Anybody is free to compete, and that's why they have many competitors. It is very common for businesses to accept a variety of unrelated payment methods. There is no monopoly here.
Why hasn't Alipay completely replaced Visa and Mastercard in US? Everyone has a smartphone to scan a QR code.