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Ah, didn't realise it was rolled to work as a full SMS replacement. That makes more sense then.
Still it'd be nice if there was an version which just simulated IM, which would presumably need almost none of the permissions
branwen-
It's "secure when it can be", which means it replaces your SMS so that if you're chatting to someone and half way through they go get Signal, it instantly starts using that and encrypting everything.
The drop-in replacement to SMS is to improve the messaging experience, and then make things really secure whenever you're communicating with someone known to have Signal.
Another misconception is that people imagine it's anonymous... anonymity isn't a requirement for privacy though. Mostly I got it because I chat with my wife and at some level I wasn't comfortable with the fact that our messages, private to us and our marriage, were essentially publicly accessible. Now I use it for all messaging communication, and if I'm chatting to someone for more than a few moments I won't continue if they're not on Signal. Crazy, I realise.
Velocio
Signal is really secure, and a damn good replacement/add-on for SMS.
It does ask for a chunk of permissions, but most of these allow it to replace the existing SMS app fully, i.e. when you start a message that you can choose contacts, that you could share your location with a contact, that you could start a phonecall (dropping back to a regular call if the recipient isn't on Signal).
The guy behind it, Moxie, is one of the greatest coders of our time, and the entire codebase has been peer reviewed by people obsessed with security and privacy.
It's about as good as you're ever going to get to offering all of the features that they do for the least possible intrusion... but because of how Android assigns permissions it definitely looks intrusive.