The game of "Golf" as defined by ball golf design has every green reachable in regulation for the intended player skill/distance level, so a potential birdie is possible by holing your putt. Every hole.
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Lots of points to address. I didn't quote all you said on all of them.
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I don't know anyone who thinks proper par is the only measure of what makes a hole good. It is a necessary, but not sufficient, condition.
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You say disc golf is all about getting birdies, but more fundamentally, disc golf is about scoring better than your opponents. Thus, holes that do not offer a chance to birdie can still offer a chance to "score" if that means getting ahead of the field. Avoiding a bogey is mathematically equivalent to getting a birdie.
Getting ahead of the competition is the game, not scoring lower than a particular number.
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"Potentially possible" is not the same as expected. It should take better-than-average throws to get birdie.
For golf, in order to get that birdie by making a putt, either it was a better-than-average putt or there was a better than average shot to place the ball near the cup.
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I doubt every golf hole is perfectly designed, but let's pretend they are.
Perhaps golf has limited their designs so that there are no holes which are not birdieable. That doesn't mean we need to. Golf also limits their hole designs so that there is almost never a need to shape a shot, or go to a knee to take a stance, or putt over or around an obstacle. Our game may be better because we offer more variety, and that may include the variety of facing the different psychological challenge of some un-birdieable holes.
Also, it may be easier for golf to create holes of the right range because golf stroke distances are not as varied as disc golf throw distances. (Not to mention the generally boring nature of their holes.) In disc golf, the length of the hole only explains one-third of the variation in scores among different holes.
I've run the numbers to see if such gaps (bad distances to avoid) are possible to find in disc golf. It's pretty obvious that hole length alone is too crude of a measure; lots of holes that are difficult on the short end of the gap overlap with holes that are easy at the long end of the gap.
There may be some gaps based on average score. Look for a paper. If the gaps exist, they would eliminate a significant portion of the holes that have been used for MPO. (Which, I guess, could be a good thing or a bad thing.)