ICQ came out in 1996, AIM in 1997, and MSN in 1999. I don't think it's fair to give the people working on this a pass by saying this is a new problem - working products have been shipping for over 20 years. Some of the devs working on Signal and Matrix were probably born after ICQ was released.

The reality is that all of these systems, including email, tend towards becoming walled gardens. Otherwise you end up with the same problem that email has which is that a huge percentage of it is spam. At this point email is basically a few connected walled gardens from the major providers. Before that the amount of spam was even larger.