Gmail had huge momentum prior to Android even being launched, though, and this in the era when Google was still Doing No Evil™. Google did begin to strong-arm their position later… but, so much of that is people’s ignorance and laziness (and that’s not meant to be criticism) - the majority just don’t take the time or have the desire to understand how any parts of the internet work, what a standard is, and why things being centralized (everyone flocking to a single email service) is actually a terrible thing.