You won't get interoperability without people creating these kinds of apps. There's no reason Company A would futureproof their platform to communicate with apps that don't exist, because that's unnecessary development; the interoperability comes after the platforms exist. Email is a prime example of this.
Sup is a New Messaging App by the Creator of Pixelfed ( wedistribute.org )
A brand new messaging app is coming to the Fediverse, and it's all thanks to the creator of Pixelfed.