I think you’re confusing monopoly power with the free market. Apple Pay exists in South Africa for example which has a 16% iOS market share, Mexico at 20% etc and only a fraction of that is going to use Apple Pay.
Apple didn’t force anyone to participate in Apple Pay anymore than Samsung did to get banks to Support Samsung Pay. Mobile payments also started well before Apple got in on them.
Banks got onboard because customers like the convenience, so if customers had a non mobile card and a mobile card many of them would prefer the mobile card. That’s not monopolistic power, that’s the free market at work.
Merchants don’t need to do anything it’s the 3rd parties making payment machines which needed to support Apple/Samsung/Android etc and they are competing in a market where Apple Pay is just another feature they can use to differentiate themselves. So again free market forces.