Sure, they don't have to re-do the backend, but there's still figuring out how to actually effect the transaction between the customer and merchant. They're already talking a 2015 launch date for this, possibly longer (can't find the article, but people involved in the project are shooting to get it done within 2 years). Changing out the front end app and store support hardware/software would just make the thing more likely to fail.
For instance, they could use a proprietary NFC solution. Works just like Apple Pay, just instead of a credit card number, it's a CurrentC account number. Sure it doesn't too well on iPhones, but I bet they could pay Samsung, etc. to include native support in their hardware, and that's a good chunk of the market.
Sure, but that's also adding several years and a huge amount of development time/risk - it's creating a whole new spec, implementing it in both handsets and payment terminals, and deploying those updates. During which time, the non-halfassed standard will be chugging along just fine, increasing marketshare and generally not sucking.
It's technically possible, but one of those things that's deeply unlikely to happen, particularly given the lead that Apple (and, very likely, Android in the near future) and the credit card companies have in rolling out a solution.
