To go x86_64-only was a mistake for Arch.

• The development team of Arch is comparatively small compared to other distributions.
• To support platforms other than x86_64 one should have access to appropriate hardware to test the packages. I for one have not had i686 hardware for a while. This is probably true for many other users as well.

Therefore, from my point of view, they have done everything right. Just like other, non-Arch based distributions, which are also now only offered for x86_64.

Distros like Fedora or Debian, or openSUSE have universal building systems and infrastructure for building packages for different architectures.

Right. And all have more collaborators and more money. For example, according to https://nm.debian.org/members/, nearly 1000 people participate in Debian.

Arch's core development team, on the other hand, consists of just 28 people without being paid for it. In addition, there are some "trusted users" (a bit more than 60 iirc) and some people responsible for support (wiki and IRC moderators etc.).

Arch just creates unnecessary fragmentation for the GNU/Linux landscape: software need to be packaged for the distro and for the same time PKGBUILDs cannot be reused in general for anything to go full Arch Linux.

Fragementation has always existed. Before Arch I had used Mandrake / Mandriva. With it I often could not use Redhat packages although they technically used the same format (RPM).

By the way, in the case of Arch or distributions based on it, you can in many cases use PKBUILD files for other platforms as well. Often it is sufficient to modify the line arch=('x86_64') accordingly. I have done this in some cases where a software for Alarm (Arch Linux ARM) was not officially offered. I simply took the PKBGUILD file from Arch Linux and changed it accordingly. And yes, this does not always work.