If English was the only language and the official language, then people would have to learn it to communicate. Right now there are so many options when it comes to language that non-English speakers can communicate without English. If English was made the only option, people would have no choice but to learn.
This would be a bad idea because setting the english language as an official language of the country would decrease the amount of people learning languages outside of english. While the legislation may force some of the Hispanic population to adapt, the overall effect on learning a language is actually discouragement. Having different languages and the need to assimilate with eachother is what would encourage learning.