The question you should be asking yourself is, "Who can be understood by more people? The one who speaks slang or the one who speaks correct grammar?"
An illiterate who invents speaking conventions can understand what I am saying, but the converse is not true.
The question you should be asking yourself is “if 99% of the people that hear it can understand it exactly as intended why is it not correct grammar?”
They aren’t inventing speaking conventions, they’re following the speaking conventions that actually get used. “Proper grammar” has always been for the upper classes, not how the masses spoke. That’s why copy editors exist, if normal people normally used “correct” grammar then they wouldn’t need editors. But “correct” grammar has always been a separate thing normal common usage.