I love Signal but this is one of many problems with centralized servers. Not only can they be disabled by the gov but they cost, as seen here, tens of millions of dollars to keep running at scale.
What is the advantage? Why are we not using P2P systems? If I can download a 30GB video problem-free over and over again, shouldn't it be simple enough to do with a 1mb text file?
A huge part of their costs is just verifying phone numbers, which is something the service does not need and shouldn't even have.