Patrick,
"Is it possible that the reason so many classic courses had more bunkering was because irrigation wasn't yet a widespread art form, blanketing the golf course, wall to wall with H2O at the push of a button ?"
What's the question? Are you suggesting that many classic courses used to have more bunkering than the same courses do now? Or that many classic courses had more bunkering than many modern courses?
In answer, sure it's possible. Is there any evidence one way or the other to support such a conclusion?
"Could that reduction be correlated to the introduction of irrigation systems and the advent of meaningful rough ?"
Do the irrigation systems at the classic courses you mention water the rough? In my area, rough isn't generally in the watering patterns. When there is a drought the rough dries up and bakes out. However our droughts usually only last a few weeks and are late in the summer after the rough is generally well grown in. If the rough is reduced in effectiveness the balls will generally run through it into natural areas that are thick and deep regardless of the drought.
"I think a case can be made that irrigated rough replaced the need for extensive bunkering."
Have you made that case? How much of the rough is irrigated on the courses you're talking about?
"Your argument is flawed because you haven't differentiated between a prolonged serious drought and a water shortage.
This issue wasn't contexted in a temporary environment, rather, in an ongoing or perpetual environment."
If you're talking about a post-apocolyptic world with perpetual drought I think you'd need to consider a lot more than bunkers to replace drought ravaged rough. Is the drought severe enough that trees die? That desertification takes place? The whole viability of golf courses would be in question. If you think of current drought areas such as AZ and Baja California, then some thoughts come to mind - desert (or hardpan) in place of rough for instance. Or alternate sources of water - waste water for instance. Or desalinization if close to the sea.
Seems to me to be a little too simplistic to equate irrigation to growth of rough to reduction of bunkering to drought to watering restrictions to reduction of rough to increased need of bunkering.