The risk being that the experience will be miserable. I can totally imagine a world where you can talk between Signal and WhatsApp, but reactions don't work, sending files doesn't work properly, changing permissions in a group doesn't propagate, etc.
Unless you force everybody to use the exact same protocol, with the exact same features, and to never add or change a feature.
Unless you force everybody to use the exact same protocol, with the exact same features, and to never add or change a feature.