UPI can be used for everything. Business and friends. And whatsapp wants to pair this with WA business (which itself is growing at an alarming rate. I am getting OTP's from businesses on WA)

If you are already using WA for sending money, why won't you use it for payments? The steps are the same. The interface is the same.

People will always use an app their friends are using. Probably the main reason why Facebook hasn't died. Network effect.