You would think that it would be beneficial for everyone to have people stick in a job role for a long time, gain lots of experience, become very good at it, and maybe even become comfortable. But just keeping things going isn't good enough in this world.
Not necessarily. Job hoppers help spread industry knowledge, which can be beneficial to everyone. That's part of what makes new employees valuable: they bring outside knowledge.
while it's true that a new employee brings in outside knowledge, it is also true that a new-comer tend to propose solutions to problems that seem obvious at first, but is then stuck with some organizational quirk or problem that prevents nice solutions from working (and find out that years ago, someone else has already proposed something very similar and it either failed or didn't work).
Old tribal knowledge is as important as new perspective.
> You would think that it would be beneficial for everyone to have people stick in a job role for a long time, gain lots of experience, become very good at it
The most standard analysis would tell you that this will cause whatever work they're doing to be done better.
But that's only beneficial if that's the most valuable thing for them to be doing. Having worthless work done well is not obviously better than having valuable work done poorly.
