I would be cautious to trust an American company just because they are non-profit. We know that if they get sufficiently big they can just transition into being for profit (see OpenAi), chances are regulations regarding this won't become stronger later.
Signal being open source of course helps, but it's usually the ecosystem that people grow to rely on that keeps them in place, not the technology. Just look at how big Reddit is compared to Lemmy for a good example of that. If signal was federal that would be quite different since jumping ship would be easy.