Pacman, apt-get, et al also have something most language specific package managers don't, maintainers. Maintainers back port security fixes and are careful not to break binary compatibility. A random $lang specific package is unlikely to have either, if you expect a bug fix you have to upgrade to the latest version. Developers that don't want to provide this sort of stability typically hate distro package managers.
No package manager can fix a cultural problem.
If you rely on dependencies that are not maintained you are already in trouble. Being the author/maintainer of a package comes with a responsibility. NPM already showed us what happens if you just let people do their thing.
And while I have been using development snapshots for years with my system and didn't have more bugs to cope with compared to other systems, I can understand that developers in corporate contexts need to be able to specify dependencies with versions. So I appreciate that proposal very much and hope it will lead to a better solution for every body involved by providing a way to use versioned dependencies without introducing too much overhead for those who don't need versioned dependencies.