"We're all going to one language anyways"
Whether that is true or not (and it's certainly not happening in anyone's lifetime alive now,) the value of having these languages not only not go extinct, but thrive, is sadly losing potency as the years wear on. This is, quite simply, because each year we're losing more and more people who know what is behind the language.
There are many, many words and phrases in any language that are not simply a way to say the same thing in any language. There are ideas, thoughts, values, philosophies - whole religions - that you can only talk about comprehensively in a certain language. My mom talks about my grandpa (whose first language was English, I might add) who would struggle to impart a Tlingit philosophy or value he learned growing up, but would throw up his hands with a, "There's no way to say that in English!"
A Tlingit teacher I had talked about one word - just ONE word - in Tlingit, "Eetoowoo" (and yikes, I think I just hacked up that spelling!) It is translated in English as "sorrow." But I can still hear her voice as she tried to explain what it really meant - it was more than sorrow. It was a deep, deep sadness that the whole body, the whole being, was involved in. Not a word, or even meaning, we have in the English language. I still don't know what she meant.
I've heard people say they can experience a culture by visiting it, by attending a dance, by reading about it - therefore why not just all speak the same language as the language isn't a part of it?
But culture isn't about attending a play or viewing a piece of art. It literally makes up who a person is.
If you think language isn't important to a culture, I challenge you to learn another language fluently. Use this language, and only this language, to your children, and forbid them to speak English. Then tell me the stories your father told have the same weight. Tell me the songs your mother sang to you can be passed on. Tell me the jokes you've giggled at since you were in high school translate to this language, and your favorite books make as much sense. I gaurantee you MUCH will be lost. Even if you're able to capture big chunks of it, there's no way to translate a whole culture into a different language in one generation.
Now try and think of this as a large group of people trying to do the same thing. Values, stories, philosophies, songs - we've already lost so much. But if we can literally speak the same language as those who can still teach it before it's too late, it won't all be gone.