It’s like saying you don’t need a version control system for coding, as you can just make a copy of your sources before making important changes.
There is no distinction between package maintainers and end users. They have the same power.
In the meantime i dont expect Debian users to ever write a package themselves or to modify one.
In nixOS you do it all the time
# clone the package definition
$ fedpkg clone -a <name of package>
$ cd <name of package>
# install build dependencies
$ sudo dnf builddep ./nameofpackage.spec
# Now add your patches or modifications
# now build the package locally
$ fedpkg local
# install locally modified package
$ sudo dnf install ./the-locally-built-package.rpm
Let me put it differently. The documentation of NixOS treats package maintainers and users as kind of equal.
This has benefits and downsides. Benefit is that everyone is treated as a power user. Downside is that power users are horrible at writing docs and this philosophy is my main theory why NixOS docs are so .... Bad
Fedora (and RHEL) end user and developer docs are written for quite different audiences
(As for why the docs are so bad, I think it's because of the lack of good canonical documentation. There's too many copies of it. Search engines ignore the canonical version because it's presented as one giant document. Parts of the system aren't documented at all and you have to work out what you've got by reading the code. The result is that you have no idea what to do if you want to improve the situation - it seems like your best option is to create new documentation. And now you have the same basic level of documentation that didn't help the first hundred times it was rewritten. And I don't really think submitting a PR to nixpkgs is exactly userfriendly, so it probably discourages people from doing the "I'm just trying to understand this, so I'll fix up the documentation as I learn something" thing.)
You're in no way bound by decisions of the nixpkgs contributors: as you say, we can add a patch. Or we can also decide we totally disapprove of the way they've configured such-and-such a service and write our own systemd service to run it.
Anyone can write a local debian package which adds a patch, and build and install it. And anyone can write a systemd service and use it instead of the distribution's systemd service. But on NixOS, these are equal to the rest of the system rather than outside it. Nixpkgs is just a library which your configuration uses to build a system.
But note that I did caveat my suggestion: "Of course, a declarative and textual rendition of the configuration is better than bash commands entered on the command line but sometimes you don't need that level of precision."