As you said, Arch could have all that but it doesn't and there are the reason why people who use Debian don't use Arch. Again, you can quickly make deb packages without following any of our docs and no, package manager should not solve what differs from our official build quality and Debian Policy (it would be too much hassle to predict and implement in how many ways people can break their system).
I am glad that Arch exists, so everyone has a freedom to choose it if they want :)
However, none of this is required for unofficial packages. You can do it, but you don't have to. This is where I'm saying you're conflating two different things - QA and packaging are separate.
Please don't get me wrong, I love debian and they do an incredible service to the Linux community with their testing. But why force it all on users that just want to build a simple package instead of doing make install as root?
Again, no one is forcing you to do official Debian packaging if you want to create simple deb for your purposes and it was mentioned in this thread by other and me several times (and several tools).
It's well known that the debian repositories are extremely well reviewed and tested. But you could have that with Arch's packaging system as well.
When someone builds a package themselves however, they are not expecting Debian's QA & testing nor are they usually interested in it. But a package, untested as it may be, is far, far better than a sudo make install.
When I used debian this was such a huge pain. Building packages was not worth the hassle so I would simply sudo make install and feel awful about it, and end up with conflicts and everything that a package manager is supposed to solve...