> 2. No multiple package versions. If there is a version conflict between transitive dependencies of a package because of semver, you should not be able to install that package.

I think this depends highly on your environment. In npm land, where a typical project has hundreds or thousands of dependencies of dubious quality, this would be a nightmare. It guarantees that every single deployment will be different and risky.

This model works much better in linux, where packages are maintained by maintainers and there is not an explosion in the dependency network. Especially on a Debian or Centos box, you can be confident that upgrading packages won't break stuff.