If the user was going to message someone off platform they’d still be sending them an unencrypted message anyways if they have to switch apps to SMS.
It sounds like they don’t want to take responsibility for that user choice or be connected to anything that happens because of that choice.
It would still be an insecure choice, even with obvious UX distinctions. It would only be a matter of time before headlines muddy the waters with “intercepted Signal messages reveal…” or “Judge rules in favor of subpeona for unencrypted Signal messages…”
Please don’t feel that the best thing you can do is shut up! You deserve a voice in the matter, even if it is much harder for you to use it the way others can use theirs.
Besides, not everyone can always be or even needs to always be the person who spreads the message. We need people to help figure out what the problems and solutions are just as much as we need people to share those problems and solutions with the world.
It is usually a necessary evil that we have marketers to sell the things that wouldn’t exist without the engineers to produce them.