I recall in the excellent series "The Story of English" a discussion of how pronunciation and word usage change in a given population over time. So, the development of "Imma" in place of "I am going to," or even "I'm gonna" is a natural phenomenon. It's one reason why we now have so many different languages.
The problem with a worldwide language such as English, of course, is that at some point, different forms of "English" may develop until they become mutually unintelligible.There are some interesting sociopolitical issues about whether those forms should be considered variants from "standard" English, or treated as different languages.