I agree about diversity in languages being a good thing. I also see minor languages gradually dying out.

First languages do change with time so it does happen, and it can be induced to happen. My grandparents all spoke English as a first language, but if you go back a few more generations only one of my great (or possibly great great) grandparents ancestors would have done.

Diversity in thought is great, but I don't think that distinct native languages are a prerequisite for that. Throughout history, the societies that were able to achieve the greatest accomplishments, according to most any measure, are those with the most people using the same language.