I've had surprisingly good luck with strong-arming people into switching. The important part is having their trust, if they don't believe you they won't listen. The next part is to make simple, verifiable, and non-technical arguments for switching. Believe it or not, almost everybody is willing to take small steps if they're free.
Instead of rambling on and on about "end to end encryption" or "double-ratchet cryptographic algorithms" or other junk only nerds care about, approach it like this:
* There are no ads, and none of the messages you send can be used for advertising
* It's not owned by Facebook, Google, Microsoft, or any of the other mega-corporations, and you don't need an account on one of their sites to use it
* It will still work great if you travel, change providers, etc
* It's much safer to use on public Wi-Fi than other services or SMS
Honestly, don't even touch on law enforcement access as in the OP. That can strike a nerve for some people. The best appeals are the simple ones.
Also, a big one that works for me (especially iPhone users, which are the hardest to convert): "You can send full quality images and videos to Android users." The fact that Apple shots themselves in the foot is an advantage to Signal.
That’s not Apples flaw, it’s a flaw with SMS. It can only handle file sizes up to a certain limit, and during periods of congestion they lower that limit.
Instead of rambling on and on about "end to end encryption" or "double-ratchet cryptographic algorithms" or other junk only nerds care about, approach it like this:
* There are no ads, and none of the messages you send can be used for advertising
* It's not owned by Facebook, Google, Microsoft, or any of the other mega-corporations, and you don't need an account on one of their sites to use it
* It will still work great if you travel, change providers, etc
* It's much safer to use on public Wi-Fi than other services or SMS
Honestly, don't even touch on law enforcement access as in the OP. That can strike a nerve for some people. The best appeals are the simple ones.