The '-in'' ending (as in "walkin'") is not sloppy speech; it's the original verb form from Old English. The '-ing' ending was originally for nouns. Until the 19th century, even upper-class speakers like Jonathan Swift considered '-in'' the proper pronunciation.
Far from being meaningless tics, filled pauses like 'um' and 'uh' serve a purpose. They help speakers with speech planning and act as a cognitive flag for listeners, preparing them to process new information and measurably improving their memory of it.
Accents weren't just a byproduct of geographic separation; they likely offered a survival advantage. Natural selection favored the development of accents and our ability to perceive them, as it allowed early humans to quickly identify potential collaborators or conflicts within and between groups.
Linguistic innovation is not a top-down process dictated by elites. Research consistently shows that new speech features are instigated by young people, women, and lower-status social groups who are more attuned to using language to craft and navigate social identity.
Classic regional accents (New York, Boston, Southern) are disappearing in younger generations. However, this isn't creating a uniform sound. Instead, new accent divides are forming along social lines like race, urban vs. rural, and even political affiliation.
Contrary to common belief, African-American Vernacular English isn't "bad" English. Many of its features, like saying "axe" for "ask" and using double negatives, were common and even respected in colonial English, found in works by Chaucer and Shakespeare.