Don't let your subjectivity get in your way. It's obvious that you're not even entertaining the upside here.

If Apple Watch users are figuring out what to do with their Apple Watch, then Apple Pay will be toward the top of the use case list, right? If Apple can have 10-15 Million (possibly more) Apple Watch adopters in 2015, then this will translate to about 10-15 Million Apple Pay users.

There's your "stampede of users clamoring for it".