That user might be in the slim minority but they do exist and I appreciate them taking that into account and taking seriously the security of people who aren't super technical.

But then the only use case is if you literally take someone's phone, switch the default SMS app to Signal, and they don't know how to switch it back themselves. If they're that tech illiterate, they'll probably call you for tech support as to why text isnt working, instead of switching to Signal. And even if they then agree to do it your way, the mythical user you described is no longer mythical, it is easier to make a mistake in the Ui for someone tech illiterate. You will often see a person's phone number in their Signal profile, after all, it's not unthinkable.

Also, as time goes on, more people start using Signal as more people see how fascist some governments are getting and how intense social engineering is with spam calls/texts/whatnot. I know a random 60 year old bus driver who switched to Signal and made other privacy changes like turning on Safari's relay because her accounts got hacked and she was using the same password for everything. To someone that is blind Googling something they don't have context for, Signal is a trusted encrypted app, therefore anything I send through Signal should be trusted. I don't think it is as impossible as you think. And again, to what benefit? There is some benefit to prevent Google possibly scanning your messages, but there are also plenty of other SMS apps which can avoid this, if it is avoidable at all.