I’m multilingual and an amateur linguist, and I could not agree more with you. It’s interesting to note that linguists don’t call dialects correct or incorrect, they call them high prestige and low prestige, noting that dialects can be associated with class and privilege.
We do have high-prestige dialects in the United States. The old upper-class, white, aristocratic dialect of the South is one of them.
That dialect has its own special rules of grammar and vocabulary like any dialect has. Yet one seldom hears speakers of that dialect criticized for their speech.
The same can be said of a certain upper-class Boston dialect. It is held in high regard, even though it differs a bit from standard English as taught in school.
Clearly, speakers of high-prestige dialects don’t get pressured to change their speaking habits, because their dialects mark them as privileged people.
When people criticize low-prestige dialects, they really aren’t making a commentary on the importance of unified language. I don’t think anybody is genuinely afraid of lack of mutual comprehension.
What they’re really doing is implicitly criticizing the speakers of the low-prestige dialects for not conforming to race and class standards.
This phenomenon is as old as language itself, and it’s tired and harmful.
English is at no risk of splitting into mutually incomprehensible language groups. But English speakers remain at risk of perpetuating racism and classism.
We need to stop.