The question is a bit vague. If the Archlinux developers stop working and take the repositories of software and libraries offline, then off course you cannot update your Archlinux based system anymore. But someone else could upload the entire libraries to a different server and these would have to be added as repositories to the pacman config.
pacman itself can be used by other distributions, if they want to use the same package format. And they do not have to access the Archlinux repositories, but their own servers in example.
That means, pacman and other package managers or repositories are not tied to Archlinux at all. So you could just use a different distro, in example Fedora or OpenSuse. These are independent Linux distros and do not rely on Archlinux, pacman or their repositories. No need for BSD.
It wouldn’t be plausible for one person to do it. You would have to maintain every single package in the repos yourself. Every time there is a new version for any software, you would have to repackage it. And you would have to test it all to make sure it’s not broken. You wouldn’t have the time to do it all, let alone actually use your computer.
Simply put, if Arch dies, we won’t be using Arch (or EndeavourOS). This is very unlikely to happen any time soon, but if it does, there are other distros.
Valve’s SteamOS, the one used in the Steam Deck, is based on Archlinux. I don’t even expect a giant corporation such as Valve would be able to develop and maintain base Arch themselves. There is a lot of knowledge, time, money and trust involved. Maybe a big company could finance it and build a new base off the legacy Arch.
I recently switched to KDE, after 2 years of Qtile. And currently I am using an auto tiling addon (Bismuth) and it’s not bad. The entire setup feels and looks almost as if I was using a tiling window manager, but most stuff works out of the box and does not need manual setup in the configs. It tiles automatically, I can use key shortcuts to do almost same basic window management and I even have multiple layouts to switch on the fly.
If you get the chance to work with KDE at some point in your life, remember the addon Bismuth - it could save your life. But man do I miss my Qtile config with regular Python code as configuration. May I ask why you have so many tiling managers installed? I would think 1 or maybe 2 are enough.
And it you prefer Gtk, Cinnamon also has all of these features with an extension called gTile (I think). I use the exact same shortcuts on i3WM, Cinnamon, and Qtile. And unless I’m thinking about it, most times I don’t know which one I am using.
There are some specific things that lets me know, though - like Cinnamon’s notifications - as I use Dunst on i3WM and Qtile.