Come on. You can say the same thing about Chrome's effect on web standards. In reality that's not how it works, and the situation is far better than Signal.
Gmail is using their influence to modify email standards[0]. This is exactly how it works in reality. If a certain service provider holds most of the user base, they can implement breaking changes without fracturing the community. In some cases it's a net positive, helping protocols evolve, and in some cases it's a net negative.