I'm not suggesting there aren't downsides. I'm suggesting that there are also benefits and that suggesting that there are only evil motives is dishonest. I'm suggesting that the conversation is heavily nuanced, and that there is a balance of harms here that must be considered.

Part of being persuasive and having people take an argument seriously is actually acknowledging all the pieces at play. Otherwise, you will find yourself forever preaching to the choir.

There is beauty in preserving language, but to suggest the 'only reason to want to decrease it is support of ethnolinguistic genocide' is quite silly and you damage your argument by dismissing them outright.

Taking it to it's extreme, there are major advantages to having one language. Near-universal communication is a very strong selling point. Having that same universal access to information and information-based platforms without having to traverse a second language would be a boon to many. Not having to spend the time in translation would increase information accessibility to many people.

Now, whether or not it's a net good, or what kind of time line that would turn into a net good is a more interesting discussion, but suggestion that there are no reasons to support the idea of a common language is a pretty dishonest argument.