One straight forward (and older) way is to charge commission from the retailers.
The digital wallets in India started with recharging balance for prepaid mobiles. When somebody recharges, Paytm and its ilk charges commission in the range of 2-5% from the Telcos.
Either way, this venture has high significance, because, the RBI imposed restrictions were severely hampering the usability of online payments, for both the consumers as well as sellers in India.
We (Razorpay) recently did a webinar[0] on UPI, along with a corresponding blog post[1]. We also have an FAQ[1] up on the same.
A lot depends on how many customers adopt UPI and how well the banks implement it. If they respect what all NPCI has said, UPI could change a lot of things (affect wallets, improve transparency, cashless economy etc). However, a lot is hanging on few points:
1. How well do banks implement UPI?
2. Does NPCI open up UPI to more players? If yes, when and who? Wallets are the one that are most interested in this. [3]
3. Do they improve on UPI over time? As of now, UPI is missing a lot of its (promised) teeth. For eg: recurring payments, pre-authorized payments, split payments.
If anyone is interested on knowing more, I could probably answer a few questions here :)
