Nearly every distro has a single "supported option"

Completely false. Debian for instance fully supports changing your /bin/sh symlink, your python interpreter and even your init system. This flexibility is not exclusive to source-based systems as you claim, it is merely Arch and Fedora that are lacking.

and if you know what you're doing (and with Arch, that should be a given), you can replace what Arch has with another option

Yes, but this goes for any distro. It doesn't matter if you can technically do it, it matters if you can do it while being supported. Nobody gives a shit if you can hack together an openrc system with Arch, because there's now the chance that your system can fail to boot after any given update with no warning whatsoever, and trying to get support for it in #archlinux will get you laughed out of the channel.

Yeah, sure, you can change /bin/sh to point to /bin/dash, but most of your system scripts will break because Arch has absolutely no clear policy on what scripts are allowed to use when they use /bin/sh, unlike Debian which clearly states scripts that have #!/bin/sh MUST only use SUSv3 features so as to maintain compatibility with any bourne-like shell. This means no bashisms, unlike what you'll find in many Arch system scripts.

A similar policy applies to any script using #!/usr/bin/python.

The problem with Arch is not what you can technically do if you know what you're doing, it's that there is no clear policy regarding what you can change without risking total breakage.

Arch does, however, allow for optional packages on many things that aren't hard dependencies

Common mistake, these are not "optional dependencies" but "recommended packages".

if you know what you're doing (again, if you're using Arch, yes, you do)

Not at all. I know some total retards that use Arch under the belief that simply by managing to follow some instructions on the wiki to install it, they are suddenly Linux gods and should be revered for their intimate knowledge of the workings of a UNIX system. Meanwhile, if you ask them about something as simple as writing a systemd unit file, they act completely dumbfounded as if you're a possessed idiot speaking in tongues.