There are plenty of unions that keep supply low. Nearly every labor union in California, for example, has underfunded apprenticeship programs for exactly this reason. I’m surprised you aren’t aware this is a phenomenon. Unions answer to their current members, and there’s plenty of incentive to keep the current membership smaller.

And my example was simply continuing the original poster’s insinuation that a union could have helped management avoid the over hiring mistake they were about to make by making the consequences of that mistake greater.