Bugs can creep into software even with the best of intentions, and it is useful for consumers of packages to have a choice whether to upgrade immediately or maybe wait until they have done some risk analysis. Also, the ability to roll back is invaluable.
I don't think forcing everyone to upgrade is going to force package maintainers to make zero mistakes.
Also the package-3, package-4 thing seems odd to me. I use Nuget and have used NPM / Gem / Haskell Stack on spare time projects. I would find it annoying to use this scheme because you have to hunt around searching in a different way to look for major upgrades compared to what you do for minor ones.
But most of all: I want my build to be reproducible, going back into history. Having the pinned version numbers committed to source control in ensures this (As you keep an onsite cache of the packages, or you trust the package repo to stay available and for published versions to be immutable).