>I'm suggesting that the conversation is heavily nuanced, and that there is a balance of harms here that must be considered.

People had to fight to preserve their language because someone said it's not worth preserving or why waste tax dollars on teaching a dying language that no one speaks when we could use it to teach something else.

There's no balance of harm to be considered when some countries like the US did it to wipe out cultures and their religion as a form of genocide. Some countries are still doing this because they view their language and cultural traditions as the only proper way of doing things.

>only reason to want to decrease it is support of ethnolinguistic genocide

Uh, you do know that was a thing though and still is in some places?

https://en.wikipedia.org/wiki/American_Indian_boarding_schools

https://www.aljazeera.com/news/2017/12/15/algerias-berbers-protest-for-tamazight-language-rights

I say the new current trend of the Russians stealing Ukrainian children and reeducating them is a good modern example of language and cultural genocide.

>but suggestion that there are no reasons to support the idea of a common language is a pretty dishonest argument.

How much knowledge was lost because people decided to force their version of one true common language on people? What stories, legends and medical knowledge are gone because people didn't want to bother to learn another language?