I think for apt/dpkg, the system definitely does benefit from decades of careful, thoughtful design— for example you're pretty unlikely to come up with a version constraint between your packages which isn't expressible using the existing Debian scheme. And when I look at various modernish packaging systems, I often see a lot of this thrown out as unneeded complexity, only to be cobbled back in later when it's discovered how essential it really is (see for example Homebrew's long transition from being a dirt simple wrapper around "configure; make; make install" to becoming a full binary package manager).
There's also the ecosystem benefit of having loads of helpers and supplementary tooling applicable to the formats— even stuff like having first-class support in proprietary binary stores like Artifactory, vs Nix where it's basically a shrug and "well... it works with any WebDAV server, so take your pick I guess?"
The main complaint I have overall with Apt is the reliance on postinstall scripts, which means that even if you download and extract your packages in parallelized blocks, you still have a long serialized step when every single package needs to spawn a shell and run arbitrary commands, even if in most cases, the commands actually originate from a semi-declarative format (debhelpers, either invoked explicitly from the rules file, or implicitly by the presence of a corresponding debian/xyz file in the metadata). Anyway, if it were possible to somehow flag packages as atomic or configure-less, it might be possible to significantly speed up these operations, especially in environments like CI where you have everything mirrored in-network or possibly even on-machine so the overall install time is dominated by the package configure step.