I agree, and I think that people are too emotionally invested in the package manager concept to back out now. I mean, for years Linux proponents have been touting it as the key advantage over software distribution on Macs/PCs.
As a Debian user, I appreciate that the stuff I install had at least gone through some minimal vetting first. And if I have to add a third party repository or download something myself to run, I'm much more likely to view that as the possibly-dangerous action that it is, and try to actively assess the reputation and trustworthiness of what I'm installing.
That's certainly not an average, noon-technical user thing, though. Vetted app stores are there to help average users avoid malware, assuming they're doing their job properly.
Things like the app store, in for profit scenarios, seem like ways to slip in monopoly control. Brew is an attempt to circumvent it.
I don't want to come across as suggesting they're a bad idea or don't have advantages, just that on balance I've always had a sense there had to be a better way.
Package managers are an easy way to do things, most of the time. That's why brew exists.
They are not a universally good way to distribute and install software. It twists the relationship between the software developer and the user.
Also, for the most part, the "Arch is unstable" thing hasn't been true for a long time. I find that it's far more stable than Debian or Ubuntu LTS on my machine because of newer kernels containing newer hardware drivers.