by assuming that all other factors are not almost identical

By using a market? ie. allowing people to bid on slots directly. I know that the context for this is "non-economic proposals" so market based solutions doesn't count, but I'm not sure why you wouldn't want a market based proposal in this case.

Exactly. They union is trying to prevent a race to the bottom!

Just because it's a market doesn't mean it will be a race to the bottom. For instance, you can set it up so that the proceeds of the auction goes towards the people taking the shitty shifts.

I think this has been studied and tested in real work places. It’s been a minute since I had a peek in the literature of workplace psychology (not since my B.S. degree in 2011) so I might be misremembering, but I think I remember the results are not in favor of market based solutions.

I encourage you browse the web and see for your self, workplace psychology is an interesting study.

Why didn't it work? The problem I see with seniority based schemes is that they totally fail to take into account degree of preferences. If you have enough seniority to have top dibs when picking shifts, and you marginally prefer the afternoon shift over the morning shift then that's the shift you'll get dibs on. Meanwhile, the less senior worker who have kids and therefore must get the afternoon shift (so they can take their kids to school in the morning) gets shafted. Some sort of system that allows less flexible workers to compensate more flexible worker would solve this.

Like I said it’s been a minute since I looked at the literature. But I think they have a few metrics to test for, including workplace morale, the distribution of how often workers get their preferred allotments, etc. I don’t remember any theory for why it doesn’t work, just that they’ve measured it and (as far as I remember) market based solutions fail on these metrics.

As for seniority rules, I don’t remember reading anything about those, but from my experience, I’m not a huge fan, I think there are better systems, particularly those which are based around consensus. However, given Starbucks’ huge corporate management, I’m guessing that seniority is miles ahead of the system they have now, which is probably management arbitrarily picking which is wide open for abuse such as retaliation. They probably picket seniority because it is very hard for management to abuse it.

>I'm not sure why you wouldn't want a market based proposal in this case.

Because a market requires certain conditions to be true: equal access to information, many buyers and sellers, and that the products being exchanged are comparable. It's possible that requirements 1 and maybe 3 can be met, but 2 completely kills the possibility of an efficient market in this proposal because there are far more sellers of time than buyers.

All capitalist economies (that I know of, please correct me if I'm wrong) employ regulation of market activity to compensate for imbalances in one of more of the three areas above. The theory for doing so is not based on goodwill to all people or something like that, but because distorted markets are inefficient and waste value.

>many buyers and sellers

To be clear, the things being exchanged here are the shifts themselves. I don't see why you can't have a healthy market between the few dozen or so people that work at a starbucks location.

There a few ways. I’m a big fan consensus driven approaches, and equal pay for equal work. A higher payed worker should have other justification for the inequality than just seniority. E.g. if they use their experience to train other workers, that in and of it self is a good justification for higher pay (as they are doing more). In a well managed workplace getting consensus for higher pay with this justification should be rather easy.

Likewise a well managed company shouldn’t have many collisions on PTO slots etc. as they would hire extras when PTO is in high demand, but if a collision occurs there should be systems in place to resolve those (seniority could be one of these systems, but should not be the only one) other systems could be some points based systems (e.g. where a worker that has previously not been favored, will get priority in collisions), or even just lottery.

As for promotions, a consensus is absolutely favorable to seniority. If a consensus is unreachable other systems could take over, but in a well managed workplace, failure to reach consensus in a promotion scenario should be really rare (and honestly a worrying sign of an unhealthy work environment).