Why would the union do this? It seems like a contrived example to prove a point. Unions would conceivably benefit from more hires because they would have more union membership.

My anecdotal experience is that unions are a massive benefit to laid-off employees. Laid off employees were given 80% pay while they waited with a known re-hire date. In other cases, they are given priority when the company is looking to rehire with an unknown rehire date. Unions have downsides, for sure, but I don't think your example points them out here.

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.

What you said does make sense though. The union apprentiships were extremely competitive, possibly because they were constrained to low numbers.