It seems to me that Arch established their convention first, so why should Arch "change back" when they where first; why are others not "changing back" to Arch's convention, given that Arch's came first?

Well, the answer is that this is a silly discussion. Arch is doing what Arch is doing because Arch is intentionally forward oriented, sometimes to the detriment of backwards compatibility with the old school (and yes, a distro not having a symlink for python2 is at this point nearly 5 years into being "old school").