Sorry if my whole post is agressive but : If you think that, then you definitely don't have much experience with system administration. Sysadmin is a pain in the ass. To illustrate, I'm working on supporting YunoHost for Stretch right now (the new debian) and just that is painful just because the version of many software changed. For instance we gonna have to totally change the fail2ban conf because the format changed. That's just one detail among other.
Now imagine with a totally different distro.
But YunoHost is free software, so feel free to gather people to port YunoHost on Archlinux.
Right now we have a hard time finding ressources to work on features that matters such as backup, monitoring, notifications, federation, "app market", documentation/guides, and so on. These are stuff that matter for people who just want stuff to work, be reliable, secure, and easy.
Being distro-agnostic is a dream. Even with 1000 people working full time on YunoHost, it wouldnt be working on all distros because they're all so different and there are too many. Or it would make the code so complicated that it would be unreadable, and apps unpackageable. Let alone the testability. Even supporting just 3 distros.
We are, and we will be dependent of Debian dev choices. For the simple reason that people want it to work on Debian. (Because that's what most people know and want).
YunoHost is not an app, it's the software that help you install apps. You can install stuff manually on the side if you want, yes. But I don't see how that's related to the discussion here.
First, that's very subjective. Also, that doesn't take into consideration that way more people are familiar with Debian compared to Arch, and that Debian is the most used distro for servers.
But in the context of YunoHost, I'm still waiting to know how would running Arch make self-hosting easier ? Does that allow to configure port automatically on the internet box ? Does that allow to configure DNS automatically ? Does that make packaging apps way easier in any way ?
Does Archlinux even run easily on ARM boards ?
Again, the purpose of YunoHost is to make self-hosting as simple as it can be, for people who just want to have a server running the apps and service they want (Wordpress, Mails, XMPP, Nextcloud, ...) without having a PhD in computer science / sysadmin.
If you care about the distro wars like as "Arch" vs "Debian", then you are not the public of YunoHost. The public of YunoHost is people that just want their server to work. Running YunoHost doesn't mean you support the Debian philosophy, it just means you want a server that just works.
I have Arch on my laptop, and Debian on my servers. It works, and that's what matters. Maybe one day I'll switch back to Debian on my laptop - that's no big deal. There's no special hell for people who run "dirty Debian" on their machines.
Society needs decentralized and democratic software/technologies, not more software that just make nerds happy.