I think you're confusing monolingualism with everyone having a lingua franca. Your argument sounds like "We should all just have one common language; therefore, we don't need others."

Bilingualism has existed in countries for centuries without either language dying out. I'm all for everyone speaking English. I want people to know as many languages as possible. What I don't want is everyone speaking only English.

It's not ok to lose some of our knowledge. Uniformity is bland, dull, uninteresting, and sad. There's a reason why we think knowing a lot of languages is cool. There's beauty to be found in other cultures and their languages. I don't want to force everyone to learn languages, but it'd be tragic if we lost French and with it Hugo, Voltaire, Verne, Baudelaire, Dumas, Asterix and the rest. It just isn't the same in English.