There have been quite a few references of "copycat", "Monkey-see-monkey-do", "If they can, why can't I?", etc. etc.

Really now. If one person gets up and another one sits down, is that going to cause a rebellion in aircraft? I don't think so. It's no different than getting up to use the bathroom in terms of disruption.

Also, and I may be wrong, but I would wager the majority of travellers in BF are flying alone or with a business associate and not with family members. So there are probably only one or two people in BF on any given flight that would even consider switching with anyone in Y. I certainly wouldn't give up my BF seat to some schmoe that I work with (unless of course, his/her name was on my paycheck) .

If there actually is a flurry of seat-swapping, then the F/A only has to ask to kindly refrain from further moving about the cabin. It can be handled easily.

Musical chairs, no. A one-time switch, why not?