Touche, but if they were there before why aren't they "here" in popular adoption? I think, although it is perfectly technically feasible in a vacuum it is not in real life.
Because it requires that all your common contacts (friends, family) use it. The question isn't whether you can do but whether you can be easier to use and offer more feature then the competition. Most people don't care. You need to be competitive.
From a technical perceptive you will have to spend more, make less, and have decreased usability for privacy, and federation. So picking any one is already shooting your self in the foot. Pick two and you aren't going anywhere.
Simplified argument but Signal picks to minimum they feel they need to do to see change they want. And I am big fan of this method. I am tired of projects with good intentions not getting adoption.