My impression is that Arch is not technically unusual among distributions, but is simply well-polished, documented, and very active (and has an easy way to install unvetted community packages). If this impression is correct, you still run the risk of unnoticed outdated software if the amount of volunteers drop, or a particularly critical one no longer has the time.

Which part of Arch's design prevents the issue described in the grandparent post? The issue is "distros lagging behind for years to provide a new package because they can't break all the things depending on the old version", which is solvable either with enough manpower or by sandboxing a la NixOS, where you can keep old versions around indefinitely for the things that need them. Does Arch use such sandboxing now?