Having a large market share of what, the smartphone market? It's the users of the peer-to-peer payment service/application that they need, and they have 0 of those right now.
Venmo already has millions of those users (many of whom who use iPhones) due to a good UX/UI and seamless integration with all major banks/card providers. So what value does apple bring to the table if my network is already set up on Venmo?
I think having a large smartphone platform to which you can roll out new features is really useful! There are what – maybe 4-5 million active Apple Pay users? That's a decent chunk of in-built market for the service, assuming it's seamlessly available to them all.
Obviously P2P Apple Pay doesn't offer you much benefit if you've already got a working solution to the problem.
I'm not sure why you'd consider this to be 'by force' – surely having a large market share, and offering a decent experience, are enough reasons for an ecosystem to develop because users want it?