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I still think what you see here is the effect of other things, one candidate being the "periodic" jobs. They are I/O heavy, so ARC (and also other caches) will want RAM. Maybe some of them also need RAM directly. Then of course, if free memory runs short, pages of all these long idling processes are swapped out first.Yes it does that - it does move out things to swap after a very long time (hours/days), so this is not related to vm.swap_idle* (which talk about a few seconds).
I don't know where this is controlled or if it can be tuned, but I would agree that it should be fixed or made tuneable: if the machine can run with this working set today, it should also be able to run with it tomorrow in the same fashion.
This shouldn't even be a problem if whatever needs the RAM would reliably give it back when needed – then, you'd of course notice when using those idle applications, but only shortly. But that's what I desribed earlier: ARC (up to 12.x) was very reluctant to really give back memory. Maybe it isn't the only example.