How can we assume this?
This exact thing has been tried many, many times - and sometimes to great success. What inherently makes you believe Apple's spin will be so much better than the other services and their unique problems?
The challenge Apple will face has nothing to do with UX - it will have to do with userbase.
It's likely (although not absolute) that using this service will require an iOS device. Given Android devices far outnumber iOS devices, this may pose a problem. Other services like PayPal and Square are universal services (device agnostic), and accounts are free (compared to Apple's barrier to entry which will likely cost you an iOS device).
Obviously we can't know for sure – but it does seem like a pretty safe assumption. Apple Pay is a good experience at the moment; it's easy to see how that experience could be extended to inter-device transfers (bring them close together, enter the amount to transfer, Touch ID and done – or something similar).
The challenge Apple will face has nothing to do with UX - it will have to do with user base.
The service will likely be available instantly to literally tens of millions of people with no additional effort on their part, as soon as Apple roll out a software update. That's a big benefit for Apple.
It's likely (although not absolute) that using this service will require an iOS device. Given Android devices far outnumber iOS devices, this may pose a problem.
I think I responded to this elsewhere, but that's only true globally. Bear in mind that even Apple Pay is only available in the US and the UK at the moment; iOS share in those countries is around 40%.
Other services like PayPal and Square are universal services (device agnostic), and accounts are free (compared to Apple's barrier to entry which will likely cost you an iOS device).
They do have the additional barrier of not being available to all users by default, though.
In the US, where this payment service would likely be available first, the iPhone has a marketshare of around 44% and growing. So the difference in marketshare is in the single digits: http://www.macrumors.com/2015/09/03/iphone-us-market-share-c...
Apple sold 74.5 million iPhones this quarter a year ago; they could sell even more this year, given the start the iPhone 6s had this year (13 million) vs. last year (10 million) during the first weekend of availability. If this keeps up, iPhone marketshare could surpass Android in a couple of years or less.
Second, just like with iMessage, Apple could differentiate between iOS users (blue bubbles) and non-iOS users with their green bubbles. Yes, this is a real thing: "It’s Kind of Cheesy Being Green" https://medium.com/message/its-kind-of-cheesy-being-green-2c...
The easiest thing for users would be to include payment in iMessage, similar to what Facebook and others are doing with their messaging apps.
Blue bubbles only. ;-)
Well, they managed just that for payments with Apple Pay -- and with the iPod, iPhone, iPad etc for their respective markets, so...
Knowing Apple's UX, it'll be rather easy-to-use if it's available – so I'm not sure that the overhead of 'figuring out if the other person uses an iPhone' would be any higher than figuring out if they use Venmo or PayPal or Square or whatever the other options are.