Who would pay
Family and friend groups whose users want a better experience and are willing to pay, or service providers who can offer paid plans.
Current workarounds
Currently, users set up XMPP or Matrix themselves, but the UX is poor and there is a lack of tools designed specifically for non-technical users, making it difficult to promote to ordinary family and friends groups.
MVP idea
It is possible to develop a clean, easy-to-use, friendly, and consistent real-time messaging client across iOS/Android, designed specifically for non-technical family users, to replace SMS.
How to validate
A simple prototype can first be created for small groups to try, to observe whether users switch; the risk lies in users being accustomed to SMS and lacking willingness to switch.