I have a different opinion (as I often do). My two cents are that manners is not something carved in stone, handed down to us from some nineteenth-century Moses; but that they are instead a changing set of rules, which adjusts to modern-day realities.
I also believe that the Golden Rule alone summarizes all the good manners one would expect from a person. Anything beyond that is, at least, artificial and unnecessary, if not worse.
What irks me the most is the whole gentleman-lady set of rules - you should hold a door, extend a hand, pull out a chair and so on, but only if you're a man, and only for a woman. My take on it is, either do it for everyone, or do it for everyone that needs it at the moment; or don't do it for anyone. But for the love of God, do not single women out, because the underlying concept is that women are helpless creatures that will fall to pieces is Man is not there to pick them up. I'm from Eastern Europe. I've seen the reverse side of this coin. When you apply for a job, only to hear: "Sorry, we don't hire women"; when your friend comes to you complaining that her husband would not do "woman's work" - read, any work - around the house - that's our door-holding chickens coming home to roost!
Don't get me wrong; these rules made sense when a proper woman was a housewife managing a bevy of maids, therefore uncapable of physical exercise (carrying parasols, opening doors) and, instead of jeans and t-shirts, had to wear crinolin dresses (extending hands, pulling out chairs). In other words, two hundred years ago. But now? Demeaning as hell. Don't get me wrong; if a guy does any of these things for me, I'll smile and say thank you, the polite and superficial bitch that I am. I would not, however, be surprised if later, that same guy treats me as his inferior.
I do, however, hear that many women actually enjoy being treated this way, so, to be honest, I don't know what to teach my sons in this regard. I just keep my mouth shut and hope they will figure the right thing out for themselves by observing their peers, whatever that right thing is.
Back to our manners. So, dynamic or static? Webster happens to agree with me:
Middle English manere, from Anglo-French, from Vulgar Latin *manuaria, from Latin, feminine of manuarius of the hand, from manus hand — more at manual
... plural : social conduct or rules of conduct as shown in the prevalent customs (Victorian manners)..."
I would opine that, since our lifestyles change constantly, then so should our prevalent customs, and rules of social conduct. If you don't do unto others what you do not wish on yourself, you will be fine in any day and age. Basically, if you don't want to see another person's half-processed food, along with their dental fillings, then don't chew with your mouth open. And so on. And that is what we should teach to our children.
And that's my answer.