It'd be a great idea, but will never happen. That's like asking all car makers to quit making cars so only Chevy makes cars.
First, which language gets to be 'the one'?
Second, what happens to all the people working with one of the other languages that gets dumped?
But so much for reality. The argument against is that some languages do certain tasks better than others.
For example: Server languages like ASP or JSP are great for database storage and retrieval. Scripting languages are better for Client-side usability and animation.
What language you choose depends on your needs. Do you need a reliable desktop application or a flashy webpage? I'd choose different languages for each of those.
It's basically a question of choosing the right tool for the job. You wouldn't paint a detailed picture with a house roller, and you wouldn't paint the house with a tiny art brush.