Honestly, having a declarative package manager is pretty important.

Consider the following: We've had the transition from Sys V Init to Systemd recently. But what does it actually mean?

It means, that instead of running a command to start a service, you now flip a switch in a clear, standardized way. The advantage is that you can get a table-like overview over all the services that are currently running. You get an overview, in other words. That is worth a lot because it brings structure and clarity into your system.

Now, with package management it's the same way. Instead of running a command to install a package, we should instead give a list of all the packages that we want to have installed, and the package manager should take care of making sure that they are installed. That would improve clarity, because you get a list of all the packages that are installed. It might also increase efficiency if you're installing many packages, because large parts of the work can be done in parallel. And importantly, you get reproducibility. Imagine you just have a file where it names all the packages that should be installed. You can just take that list and copy it to another machine. Now you've cloned your package installations. I guess things like Docker, with their docker files, are kinda already going in that direction. But it would be nice to have support for it in the mainline operating systems.