The only answer is a fundamental infrastructure shift that is standardized (ie. RCS). I jumped on the Whattsapp wagon back when I had an iPhone 4 because that was what everyone around me was starting to use. Whattsapp had a big enough market penetration at the time to be ubiquitous enough to be worth moving to.
If Blackberry had allowed BBM to be installed on any device at that time I dare say that everyone would have moved to that platform around here. Unfortunately they waited too long and Whattsapp was the go-to option in Canada at that time.
This argument really boils down to folks who feel comfortable moving from one messaging app to another, to those who just want one solution and have it work. A lot of people won't understand the other's point as being important enough to them because its an underlying value issue.
I have no doubt that we will move to RCS (or similar) worldwide as an infrastructure upgrade that we will all end up using and tech enthusiasts will look back at the messaging dilemma as a quaint relic of the smartphone era. The issue will be that how long will this take.
Until then, OP won't have the issue resolved.