An interesting question indeed! A question that's quite complex too for some people (given certain circumstances).
However, I believe most of the question of "par" itself and the question of changing par for a hole or course is not much more than "perception".
As was mentioned above the concept of "par" is essentially the baseline on which the entire handicap concept and the entire handicap system is built. If "par" could be looked at more realistically and more accurately that way, I don't believe there would be much resistance to changing par on CERTAIN holes and courses.
Again, the entire handicap system and things like "course rating" and even "slope" and handicap hole allocations are based on the concept of "par".
For some reason increasing numbers of handicap golfers have come to believe that par is something normally acheivable for them! Nothing of the kind. Hard par holes are intended to be "bogie" holes for the handicap golfer.
Probably the simplest way to break the prevalent misperception that slope is the difficutly of a golf course for everyone would be to publish the "Bogie rating" that all course raters establish to create the "slope rating" (by subtracting the "course rating" (what a scratch player is expected to shoot) from the "bogie rating" (what a bogie golfer is expected to shoot)).
Also for some reason too many handicap golfers have come to think of the concept of "strokes" (the holes strokes are allocated to them) as the holes that they're supposed to have an advantage over a "scratch" golfer. Again, nothing of the kind! It's those holes that a handicap golfer needs strokes to "equal" the score normally made by a scratch golfer.
This is also the reason why some golfers assume that very difficult holes (against par) are the holes they need the strokes on. Again, nothing of the kind. The reason is often these are the hole where its harder for the scratch player to make par than it is the handicap player to make bogie--hence there is not so much need for a handicap stroke.
(By the way, Tom Doak, hole #4 Stonewall is probably the most realistically handicapped hole (I believe #1) I've almost ever seen although very few golfers understand why it should be that low!).
But as to the perception problem of dropping par on a hole or a golf course-that can be complicated in certain circumstances. First you have to have a hole where that would be easy to do without redesigning the hole somehow. Certainly not all par 5s are good candidates to drop par on. Pine Valley certainly has none and either does Merion. But NGLA, for instance has at least two great candidates to drop par down and maybe even three without doing much of anything to the hole (other than moving the tee blocks a little).
I don't really understand the fixation of having to have a par 72 course either. I think one of the main reasons to have par 72 for some of these new construction courses is it's logically easier to get a par 72 course to or over 7000 yards (without facing the lack of variety problem) than it is to get a par 71 course and certainly a par 70 course to or over 7000yds. I think the total card yardage thing is the biggest reason that people think of when estimating difficultly (not necessarily the par).
Frankly to me the par 70 courses are the hardest of all or certainly seem to me to be (although other golfers seem to view it in the reverse!).
But apparently when the recommendation to drop par on a hole and course is made it's generally the handicap golfer that resists. For some reason they think that's making the hole harder for them and not the scratch golfer when the reverse is true. And that generally will be borne out by the fact that those holes are the ones that will then go from the highest shot holes to the lowest shot holes, something that the handicap golfer should be glad for! He won't score any different on the hole and either will the scratch player but he will now get a shot on the hole where he needs it more to even things out!
But basically all this stuff of dropping par on holes (and courses) that need to have it dropped are problems of perception--or I should say misperception. And that's the primary reason there's resistance. Educate everyone to what par really is and is supposed to be, though, and that resistance should go away or minimize.
I must admit, though, that those courses like NGLA and Maidstone that are par 73 and 72 respectively are by far the best candidates to have their pars dropped because they both have par 5s where that would be very easy and logical to do (again without redesign) and the also have "extra par numbers" to play with (to drop) to get to 72, 71 or 70!
But other courses like Merion and Pine Valley (and other present par 70s) don't really have that option to drop their pars because if they dropped to a total par with a "6" in front they're going to have another massive perception problem of being considered too easy!!
Again, another misperception but one that would be near impossible to overcome, in my opinion.