LLVM was held back for a good reason, it was breaking things left and right. Even so, if you really needed it there were always AUR packages for it, or lcarlier’s mesa-git repo if you prefer prebuilt packages, so it’s not as if you were just SOL. I got my 7900XT in december, and instructions on how to get it running were already all over the forums and subreddit at the time and it was working on the same day that I got it.
I don’t know when you got your 7900XT, but it was broken on Ubuntu too for a good while, I’m not even sure that it currently works on 22.04 without using external PPAs. In the mean time, it now works with Arch out of the box.
As for the grub thing, I’m not sure how that could have been handled differently. Upstream introduced a change that created a compatibility issue, so Arch could either not update to a newer version of grub ever, or update anyway and tell its users how to handle the compatibility issue. The latter is what they did.