I mean, you don’t necessarily need a fresh Arch install; I’d wager you could just switch to Arch in place by editing your /etc/pacman.conf and /etc/pacman.d/mirrorlist to Arch repos. You’d probably need to keep a Manjaro section at the bottom of pacman.conf to avoid breaking too many things at first. Is this a good idea? Probably not, but it’d be fun to try.