I think this would create a greater unity among all humankind by making communication with different people much easier.

I understand that, to an extent, culture is tied to language, but many people of other cultures still practice those cultures while speaking english. It doesn't seem like other cultures would disappear if all people spoke one language.

In fact, the fact that language is a part of culture seems to provide even greater evidence that speaking one language would bring the world closer together, by making all cultures that much more able to understand/empathize with each other.

Edit: I am not arguing for one particular language over any other. Also, this is regardless of how everyone comes to speak the same language (maybe starting with everyone speaking it as a second language would work best?), I'm just arguing that if they did, it would have a positive effect overall.