I guess yay is an easy solution, but it's not very clean, at least from what I remember and just checked. It might be fine for single machines, but since it doesn't build in a clean chroot, you can never be sure that the claimed dependencies are actually complete, and as such, a package built with yay on one machine doesn't necessarily work on another, even for the same processor type (portability might not be possible anyways if you build with -march=native). It also doesn't handle automatic rebuilds for necessary .so-bumps, but this is generally non-trivial to solve AFAIK.

When I still used Arch exclusively, I had my own repository set up via aurutils on a remote server, granted this doesn't handle .so-bumps by itself either but at least you get somewhat clean packages every time, and you'll start to notice how many AUR packages are actually broken, with the most common occurrence being git not listed as a makedepends for packages that retrieve their data via Git because everyone using the AUR has it installed anyways to access anything on there. Granted this is a non-issue in practice but it's not the only one.

I agree, but using yay, or rather the AUR, means being forced to use Arch. That's not only annoying for the average Debian/shopless-distro-user that does not want to relearn their system, or sysadmin who does not want bleeding edge software to host their website (as it may be your favorite machine learning 'anime' generator that's going down due to Nvidia drivers). It's also deadly for the 69 year old grandma as she somehow manages to use flatpaks (or whatever) on Ubuntu, but forgets to update them. Meanwhile she, very consciously, updates everything else through the Software Center every day (or lets it auto update). She wouldn't survive that jump to Arch (and certainly wouldn't survive the compile times of some AUR packages). Everyone suddenly using Arch would crash the whole ecosystem and community. Sysadmins would need to switch to Arch quickly now, as it's development stales because no one uses it.

The only solution would be, to create - yet again - a universal alternative to the AUR. Maybe someone, backed by eg. the Linux foundation itself, could create a good way of compiling AUR packages on every system. Now we would still have to somehow drop Flatpacks and Snaps (especially the latter), which some Distros will refuse to do. Canonical isn't going to yeet snaps out of the Store because it's shit and something better exists (because that would apply to the whole distro /s)