You said yourself, big companies don't want a single messenger app to exist, but you don't say how you'll get around that. How exactly are you planning to get around the "big players" banning your API keys?
Our workaround will be working with the networks we can. There are hundreds of messaging apps boasting over a million active users. If we can get half of the top 100, we become dangerous. Once we get that major market share, we start to make partnerships to get the integrations we couldn't before. While we do that, we will begin to unify other things like email, phone calls, and video chat.