What is the “correct accent” for English? Even within America there are so many. The southern accent is so different from the Minnesota accent. Most Americans will have difficulty understanding a Scottish or Irish person speaking with a strong accent, but I doubt anyone is going to tell them to speak differently. Given the plurality of accents, it’s on the listener to adapt. Unless, of course, everyone is expected to speak with RP.
Is there a standard measure of how “understandable” an accent is? It is quite a subjective thing based on where one is from.
You mentioned India previously - there are 350 million English speakers in South Asia (with marginally varying accents) who can understand each other perfectly well. They may not, on the other hand, find it as easy to understand American accented English. Who should change?
I find German and Singaporean/Malaysian accented English easier to understand than most American accents, because they share phonemes with the languages I speak. Which is more understandable in this case?
The assertion I’m challenging is that there is a “correct accent” that is universally intelligible to all, especially for a language as widely spoken as English. I think the only way we can bridge this gap is to be better listeners. Realistically, it doesn’t even take a couple of weeks to become comfortable understanding a different accent, probably much less if you pay attention. Personally, I find this issue to be very intertwined with the tolerance we have to develop to live in a multicultural society.
You said you were American (though it’s not clear if you work in America, so forgive the assumption) but if this was official feedback then it seems to be in violation of Title VII of the Civil Rights Act. There seem to have been successful lawsuits (example, example - see Brown and Brown Chevrolet, 2008) for the same.