Any single widespread "common language" will tend to diminish the home languages. A contrived language like Esperanto has no utility at all because it cannot become widespread lacking some all-powerful ruler who would decree that only it be spoken and written. The lingua franca must be naturally the language of the dominant group because that is the language that the movers and shakers, especially the commercial movers and shakers, use to move and shake. French never replaced English in England because, though the elite after 11066 spoke French, the traders did not and the French-speakers were not traders.
How come diseases are always "brought in" but the natives never have diseases of their own?