What will you do when the underlying networks start to work against you and shut your third-party client out? They don't want you on their network, they don't want people to use your client, they want people on their client, so they get all the metrics and data.
Look, I had a multi-network IM client for a long time, because screen real estate and notification area icon space was limited, and it was nice to have similar-looking dialog boxes for everyone. But those aren't problems on mobile.
I get it, it would be nice to have the app that everyone has installed, but what problem are you solving for average people? Why would they install your thing and not the real thing?