even when we make the most diabolical blunders in the use of the language, we seem to be able to understand each other on the surface.

Which pretty much limns my take on opposing or defending linguistic change. There is no confusion about meaning when someone says, "I seen that movie arready." If someone (mis)uses, e.g., the word virulent, on the other hand, there can be much confusion (Wah, Cuhnel Beauregawd, you are just so strong and virulent!). But we learn to live with it and, if a distinction in meaning is required we will develop other means to express it.

"I seen" and "I been" seem to have more origins in African-American and poor Southern speech patterns

And it is exactly these people who will have the least opportunity to relearn formally the grammar they develop informally.