Since we are discussing how to define "par," I have done a short bit of cursory digging into the etymology of some terms to see how they came to mean what they do now, and I found the results interesting enough to share.
First, let's look at "golf." The earliest mention of people playing a game like golf dates all the way back to the 11th century. Most references are to something called "kolf" (or "colf") which essentially means "stick," "club," or "bat." These references indicate using a stick of some sort to hit a ball towards a target, but they are very unspecific. The modern game of golf is an invention of the Scots. The earliest Scottish reference to gowf is from the 15th century, but the oldest rules we have are from 1744.
I could not find the history of using the term "scratch." It probably comes about by defining a starting line (a metaphoric scratch-in-the-ground to define a baseline of play), or maybe it refers to a tee-line (a "scratch golfer" would be able to reach the hole from the scratch-line.)
The term "par" is a derivate from the Latin "par, paris" (meaning "equal" or "like"); even after the fall of the Latins, "par" was still used colloquially. It was not used to describe golf scores until the end of the 19th century. Around the same time, there was another term being used to describe "scratch" play: bogey. A bogey score was the score of a phantom-player (a bogeyman, you might say.) The abstract goal of golf was to beat the bogeyman.
In the end, though, "par" became the term to describe playing scratch. I found no evidence, but I suspect the reason for this is that scoring under-par was very hard. People did not like losing over and over to the bogeyman, so they made him play one stroke worse.
A recurring point in this thread has been the idea that people only want every hole to be birdie-able to stroke their own egos. If my above theory has some truth to it, then it means (funnily enough) that that the modern definition of par and bogey, which some want to hold on so fast to, is the result of ego. This is not verifiable, of course, but remains amusing to consider.
Using the term "birdie" to describe scoring under-par did not seem to start happening until the mid-20th century. I could not find where it came from (apart from a couple legends,) but I think I can make a reasonable guess. As has been stated, birdies in golf are hard. I suspect that in the past they were even harder, but they started to become somewhat more commonplace as the tools we used evolved: better clubs, better balls, more manicured courses, more shared techniques. As under-pars started to become more and more common, people in different areas started to call them different things. Someone decided to call theirs a birdie, and other people liked it so much that the name spread (let's be honest, "birdie" is a really fun name.)
There are many things you can infer from all this information. The most important conclusion to me, though, is that words can change meaning. Disc golf involves zero clubs (except for the ones I throw into trees to get my discs down because I suck,) yet we still call it golf.
Trying to strictly follow the meaning of words is a bit of fool's errand. The proper course of action is to determine the best, most reasonable, most practically useful way to track scoring and go from there.
To me, I don't care about how the bogey-man/par-man is doing. I care about what the best possible score is on a hole and how close I was to it. My struggle is not against flesh and blood, but against the trees, against the rocks, against the whims of this windy world and against the physical forces of gravity in the disc golf realms.
