Consider this: each additional language improves humanity's capacity to evolve and shift, a particularly valuable trait considering the pace of environmental and socioeconomic change. More languages increases the likelihood that humanity will continue to survive and maybe even thrive. (Reality one hundred, one thousand, and ten thousand years from now will each demand different models or languages.)
I am personally doubtful that humanity will be able to succeed for more than a few hundred years as a mono-culture. It needs variance (multiple languages) and experimentation (new languages) to evolve and respond to change.
My argument is not about individuals speaking multiple languages, it's about different groups of people adopting different models of reality and trying them out.
Saying there should be fewer spoken languages is frankly a very unempathetic thing to say.
Having less major cultures has the same effect on potential conflict
You're basically asking people to sacrifice their lives to a less useful language, a mere social utility, so you can maintain a sort of "cultural linguistic zoo" ... and yes you are by this analogy relegating them to the role of animals in a theme park.
Think of what that entails: "No, you can't be taught in the language that will get you into university and get you the best jobs. You must dedicate a significant portion portion of your life to something we already know is a dead end for you personally, because we have decided that it is more important that a legacy tool be kept alive than that you prosper as a person."
You've inverted the role of language. Language is what we use to communicate the things we feel, we gain essentially nothing from the words themselves. Even when you think you're communicating something very specific with the words you are using, there's little reason to believe that the person on the other end of the conversation really gets what you're trying to communicate. In a practical sense, you can easily communicate, but these deep, almost mystical, meanings you're implying... it's just not really something that can be known.
The is the point of the indeterminacy of translation illustrated by W.V. Quine.