I'm not out of the industry. Still heavily involved in it. The non-union workers aren't working with a gun to their head, metaphorically or otherwise, they are working hard because they know that being better at their job than the next guy will help them when it comes time to promotion, raises, as well as layoffs. While at a Union shop all you need to do is the bare minimum to keep from getting written up enough times to get terminated. You'll get the raises and avoid the layoffs no matter where you fall in the performance category, strictly based on seniority. If seniorty went to the wayside and all raises and layoffs were based on merit, I'd bet the work ethic at many union barns would change. That says it all right there.
Who would decide what was meritorious? How would merit be measured?
The reason for seniority was low level supervisors playing favorites based on factors not related to the job.