When I say Arch is not stable, I don't mean that you can't leave it running for a long period of time. I mean that it changes. Debian is not stable because you can run it for a long time without crashing (You sure can, but you can also run Debian with daily crashes, depending on what you're running). Debian is stable because it doesn't change. You get security updates, but you don't get feature updates, because feature updates introduce change, and the way you thought something was done is not the way to do it anymore. Flags change, output changes, inputs change. None of this is bad, but with Debian you know it won't happen until you're ready to move on to the next version. "Unstable" distros can introduce these changes at any time, making it harder to review what will change.