Also to your other part about "it's not that hard"
It actually is. We cannot get enough bodies in the mill to not work everyone at full time. We pay 18 year olds 6 figures to operate a mill and we still cannot get enough bodies to come anywhere close to working a 4 day week.
It straight up doesn't work that simply when you're running enormous 24/7 operations in critical industry. Thats like all the football fans on the couch yelling at the coach "JUST DO X". Really easy to say "do X" but the application becomes extremely difficult. Yeah in an office, sure....in a refinery where you create base stock products that allow hundreds of other major plants to run to produce all the basic products you use every single day? Not gonna work that way
This isn't some machinist shop that takes orders. it's a multi billion dollar full rip steel mill/refinery/plant/etc. that loses LOTS of money when it's not at full capacity. That has lasting knock on effects on other industries for example when base manufacturing can't keep up
The mill I work in is already set up with shift work....they work 1st shift (week 1), 2nd shift (week 2), 3rd shift (week 3), and then go on long weekend 4 days off. My department runs 21 turns (we are the plant bottleneck by design), meaning 3 shifts of 8 hours, 7 days a week. Most people dont work 7 days straight unwillingly, but regardless of that fact, you need to keep running. Not running is losing money, losing money gets corporate to shut you down, getting shut down means you have no job and the company doesn't care either way
There is a trade off when dealing with continuous operations. You run into the issue of, "Not running costs more money than running and paying people overtime." Moving to a 4 day week just means you would likely get forced more into overtime so we can keep steel flowing, not that you get more free time.
Also from the salary side of things, I just spoke to 4 other process engineers and all of us immediately agreed that we cannot get the work done required of us + do the extras of being a floor process engineer in only 4 days. We could get our "requirements" done, but then all of the extra work that we perform would cease. It would actively hurt the company and its profitability, which in turn hurts our job stability. Its really not as cut and dry as people want to make it seem in all instances