There are a number of good alternatives. Signal wins because it’s well known, easy to use and install. They are targetting private communications, not a specific app.

It’s all very short sighted. If you really want to stop private communications you have to outlaw all people with technical knowledge and access to general purpose computers. I can cobble something together that is secure enough for a criminal or terrorist to communicate with freely available software but it won’t be full featured of nice to use.

Taken to the extreme this thinking ends with sending all the people with glasses to “work” some fields in the country because intellectuals challenge the security of the regime. That makes no fucking sense in a liberal democracy. So why even start down this path. Get a warrant and surveill people at the end points. It’s the only acceptable solution.