Unions do not exist to protect potential members, they exist to protect actual members. Saying "it's harder to break into the industry" isn't going to discourage someone already in the industry.

This actually does hit on the major difficulty in unionizing, which is that people want to make games. It seems fun. It feels rewarding to point to something bringing joy to your friends and family and go "I helped make that!" Because this feeling is so common, it's extremely easy to find someone to to do the job for less. If you're a cost minded manager (instead of a results minded manager) you might be willing to take on 3 dirt cheap juniors to do the work of one really expensive senior dev.