Yes, that's actually quite good argument when not everybody can just change their messaging app just because their contacts won't use it... :|

A wild idea would be that some new alternative app could communicate with other messaging platforms via central server that does all protocol conversions etc. (this could work with in-app plugins too, no need for conversion server).
So at the start the app would only support Telegram and support for WA is added to the plugins/server later.