I think that in the long-term it might be better if English didn't become the main language of the EU. It ties us too closely with the US culturally, because the US is a massive native speaker of it.
I have no idea what we would replace it with though. Everything seems to have massive problems.
English is already the de facto auxiliary language of the world. The point of a language, above all others, and especially so for governing a union of countries, should be maximising the ease of communication.
And in the process it kills off the smaller languages in said union along with their culture? Look at France and what happened to their local languages in just a few hundred years.
I have no idea what we would replace it with though. Everything seems to have massive problems.