When it stops being okay is when Signal's know-better-than-you/opinions are motivated by their commercial interests, and in conflict with the interests of the users.
The "interests of the users" is not a single, homogonous thing that you can point at and define. Some users' interests conflict with other users! Also, interests can have a whole range of motivations, and I'm not sure any single class of motivation (commercial, spiritual, whatever) objectively outranks any other. People are going to disagree on that, because everyone ranks values differently. I don't get to tell Signal what to do though, they should be free to balance interests however they like. i may not fully agree with their approach, but may also choose to compromise on that because there's nothing out there that fits any better.
This idea that "someone built a thing, has strong ideas about what that thing should look like, and some people don't like that because they don't share the same values" being somehow "not ok" is peculiar to me because it's at the very heart of fundamental freedoms around speech / association etc. If you don't share Signal's values, or disagree with how they balance competing interests, go use something else!
