It's still a package manager, and it still has dependencies, but the dependency chain is "flat" such that it's always comprehensible and you'll never be stuck modifying large parts of your system to update one program. I see it as similar to Windows's Visual C++ and DirectX runtimes in that regard.

I hear you on installing to different disks, though. Appimages are also an option, but I've found they have a tendency to not actually work between different distros and system versions.

Unfortunately that's a problem with they way Linux is structured, or rather it's lack of structure. There's no consistent base system that can be relied upon like you have with other platforms. The community seems unwilling to deal with this issue, some outright deny that it is an issue.

One thing that would be interesting would be to use flatpak "runtimes" to provide a target base system for AppImages, but I don't think that level of cooperation is actually possible within the Linux community.

cough LSB cough

I don't really think it's an issue though, if you really want portability you shouldn't rely on any software provided by the distribution unless you want to depend on that specific distro.

Flatpak runtimes are basically interchangeable base systems, it seems silly to commit to any one in particular when you can use any number in parallel.