Like many situations in our society, I think this is one where we should rely on self-policing and common sense.
We'd all agree there are circumstances where someone from coach has a pretty good reason for breaching the curtain (blocked aisle, broken lav in back, length of line combined with desperation, FA told 'em to). And we'd all agree that there are a few fuggheads who'll try it just to prove they can get away with it.
So? Creating rules and enforcement mechanisms will just diminish everyone's experience in the long run. (Personally, I think most of the rulebreakers do it hoping to be challenged, to make a scene. When they're not, all they did was cheat & get away with it; if they enjoy that, they deserve to live with themselves.)
I suppose how much it affects FC pax depends a lot on how many fuggheads there are, and how assertive they are.
Compare it to a closed lane on the freeway. (We drive a lot, too.) In some parts of the country, a sign two or three miles ahead leads 98 % of the drivers to line up in the unrestricted lane well in advance. You may slow up a little, but the few idiots who try to horn in at the merge point don't cause much trouble. Everybody gets through okay.
In other places, 20-25% of the traffic tries to take advantage of the "faster" lane, and everything comes to a screeching halt a mile or so before the merge point. There are too many cheaters to absorb, so everybody gets punished.
The rules are the same in each situation, but the behavior of the drivers is different, leading to different outcomes. I don't think stricter rule enforcement on the plane would help in most cases, but maybe some of you are on flights where this could be an issue.
Just hope you never get on one of those flights where the food is "off". The lav situation has no class boundaries then!
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"Service" should be a noun, not a verb.