I think you read far too much into the advice. You go so far as to suggest a racist inspiration for the comment.
Yeah, I'm sure that's what Ms. Cusack meant, that blacks should not be in films. Get a grip, huh?
Maybe she was talking about the industry itself, not about the young, beginning actress. Show business, including the movies, is a very tough career field.
We only have a second hand account of the exchange. It is not beyond the realm of possibility that the remark was made as a joke - Joan Cusack hardly fitting the idealized Hollywood image of a female star.
Have you never heard nor seen someone advising a newcomer to any business "Get out now while you can. this business will kill you." or words to that effect?
Acting is easy. Making a living acting is extremely difficult.
Your article is contradictory to me. You say "Rejection and hurt are part of life." in an article complaining that a person didn't keep to herself a comment that that you suggest could be taken as rejecting or hurtful and suggesting that others should not make comments or give advice that might be discouraging.
Some years ago a friend of mine said he was thinking of going into air traffic controlling. I said he was nuts and didn't have a chance. My significant other at the time took me to task for discouraging the fellow. He was a high school graduate who had made C & D grades and had been a significant user of "soft" drugs for two decades. Perhaps I should have advised him to follow his "pipe" dream; I don't think so.