I was able to get grandma on Signal precisely because it was the little secret between us. She knew that a signal coming in was from me, and only me. I was using it to talk to her. And she thought it was cool that we talked via signal to her before any other family member caught on the signal craze.
I was able to get my SO to use signal on the same principle. Pick the people in your life that care about you - and care back. You are available via signal, while you ingnore whatsapp et al. You answer right away on signal whereas on other stuff, you wait.
Its funny how far along you can take a feature-lacking app provided its simple for others to use, and there is an emotional attachment in its use.
I just told her to dowload the app.
At its core, signal is still a messenger app. Its not hard to use. I dont think you realize how powerful are simple things like "good morning" and "i love you" to your family and loved ones. Give a little of it, and see the results.
In the local and direct sense, it is no worse for grandma, and that is the point everyone is arguing. It will work for her use case while also being better in the global sense.
I'm also a privacy advocate, but there are downsides that you need to acknowledge.
I would however have trouble with the implication that having this stance is the consequence of being in an idealistic bubble, though perhaps you did not mean that.