You are the one introducing "monolingualism". In no way not saving dying languages will lead to it. Even if by some act of god suddenly everybody learned english and forgot their own language in a night, very soon you would see emerge variations, slangs, then dialects. Languages are like living creatures, they evolve.

My point is mereley "we got thousand of languages, it's ok to loose some of it, let's spend more energy on making sure we got at least one lingua franca".