But how is it better for Grandma? Does she really care on her own or did you talk to her until she accepted that she cannot receive messages from you unless she uses Signal?
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 global and indirect sense, it's for better for grandma in the same sense as it is better for the whole of society and the entire world. Privacy is something well worth preserving instead of selling to a large corporation and/or overreaching governments.
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.
If you need to keep conversations across 2 messengers, that's at least less convenient. But noone seems to see that and everyone else around here is living in their idealistic bubble.
I'm also a privacy advocate, but there are downsides that you need to acknowledge.
Sure, I acknowledge this as a (small) downside, but it is at least no larger a downside than not having control over your conversations or being part of a dragnet to be spied upon by corporations and governments alike.
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.
Signal is an E2EE messenger app with as little shared metadata as possible. Unless Grandma uses the "Status" feature, or wants to know at what time a message was received or read, it provides the same features as Whatsapp with an increase in privacy. It's fair to say that it's a better alternative in her case.