Got to ask now: are you working for Signal? It really sounds like it....

Re sysadmin and self hosting it really depends on the scope. You can follow basic security recommendations, which you will set up once, and which will protect you from bots and scripts. What about targeted attacks? Not sure, but if you host for yourself or family and friends only, I wouldn't take this as my threat model.

I have a Nextcloud server running for about 4 years now with close to zero downtime. During those years I had perhaps two instances where I did larger upgrades which took me perhaps an hour on two weekends to prepare (basically backing stuff up, dealing with the excitement, reading up) and then maybe one evening to execute.

I'm sure hosting for millions of people who might end up sending lawyers your way is difficult. Running something like matrix yourself for friends and family simply isn't and it removes all the security and privacy risk that comes with trusting an US based organisation that had its share of controversies, and more importantly is subject to the questionable laws and enforcement practices.

Anyone who had installed linux and is happy to work with a console would be capable of learning how to self host in a reasonable amount of time.