I don't think anything will ever change that. The problem isn't WhatsApp but the general problem of "each person is only reachable via certain contact methods" which can't ever be solved by anything.
The DSA forces platforms to allow federation and to expose public API endpoints.
So it solves this because it makes it possible for your to talk with someone on a different app.
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.