I don't recall much discussion on the theory of bunker depth in any of the books I have read, other than fw bunkers should be shallower than greenside bunkers. And yet, it seems a subject worthy of scholarly discussion and debate, no?
Did the early Scots really think about bunkers or just accept what the sheep dug? Most TOC bunkers are similar depth, a function of how low a sheep digs to stay warm perhaps? (like railroad tracks being 4'8.5" wide because that was the width of Roman carts, are bunkers unconsciously as deep as a sheep burrow?)
While bunker depth has to follow the land in most cases, there are many pictures of "built" bunkers to suggest that its not just a function of land in each case. Did the Golden Age guys just dig until the horse gave out?
Or did they think about the theory of bunker depth? As mentioned, Ross plans usually had a +2 or -4 depth indicator which might suggest a penchant for moderate bunkers. MacKenzie (my impression) was more interested in the art of his bunkers, and more depth allowed more interesting shapes.
Now that bunkers are more easily built with dozers, it seems like perhaps we can give it more thought. For all the words devoted to the subject of bunker placement, shouldn't there be at least some on the subject of bunker depth? Its an equal component of how they affect the game. For example, the gca places a bunker on the inside corner of the DL to challenge/encourage a shortcut. Do players react differently if that bunker is 2 or 20 feet deep? I think so!
As to the overall ease of bunkers, you could look it up in my previous writings, but I have speculated that basically, as life has gotten easier (when was our last US potato famine?) people generally want sports easier. As we can predict things (weather, etc.) better, people want their bunkers more predictable, etc.
Then there are the obvious speed of play/average golfer difficulty issues. While Doak seemingly dismisses myself and John's questions by speculating on the Scots, his idol MacKenzie did write something about golfers not "piling up" huge scores as well, and was part of the softening trend for this reason.
As John says in the original post, there is the attitude that the bunker shot isn't really penalty, its an ability to create/play a different type of recovery shot. Among them high and low, high and low spin, shots into banks and trail away slopes, and even putting or chipping out of the bunkers.
Of course, add in fw chipping areas, grass bunkers, etc. and it seems that over the course of a season at a course with a wide variety of hazards, the the player would have the fun of playing a wide variety of recovery shots. So, generally, variety is good for everyday play, no suprise.
I must go, and look forward to related thoughts. Sorry for the rambling thoughts.At some point, maybe I will have time to codify.