If we got down to only one language, that natural human tendency would be for new languages to grow out of that. They naturally change all the time, and suppressing that is a loss, not a gain.

Hmmm maybe. There are obviously benefits to adapting languages to better describe the environment the speaker finds themselves in. But the utility of being able to communicate with others (especially if they are the overwhelming majority) is probably higher. In the absence of geographical separation, I don’t think new languages would have become mutually incomprehensible.