Seniority is important as it makes a fair playing field. Seniority doesn't guarantee you the job, it just gives you a shot at to qualify. You still need a work ethic and some level of common sense. If you don't have that, seniority won't matter. A "good management " team would be able to weed out the "slugs" from the actual hard workers who could do the job correctly.
And what constitutes a "hard worker"? What you might feel is hard work might be less than what someone else does. Seniority protects against that in many ways. The guy in his 50s might not work at the level of a kid in his 20s. Should he be thrown to the side even though he has dependability and experience on his side? No.
When you work here long enough, you learn to work smart not hard. Seniority has its place and should be honored. And if you think the "lazy" workers are just in union jobs , well you'll be disappointed to know they're everywhere.
As with everything, there should be a balance. But seniority does have its useful place.