"Young actors with minimal experience will jump at the chance to join the union. Many of them will not be getting union work, and now they won't be allowed to audition for nonunion jobs either, so they will find themselves not able to work anywhere."
As you and others in this thread have said, this could be a real problem, and that problem could disproportionately affect people of color. Jennifer Paz, who is Filipina-American, is a well-known example of someone of color who left the union because of the lack of union gigs. She was around 21 when she was the alternate lead in the national tour of Miss Saigon, but she later left the union to take the role in a non-union production. Part of the issue was that she felt there were few union opportunities for someone like her. That scenario could play out more often now.