Your question makes sense, but I think you're right - It's not that simple.
Your theoretical Debian distro might be fine for general purpose use, but there may be people who want a distro for games, or a wide range of hardware compatibility. In both of those cases, you might need to have a kernel compiled with specific options, and a distro bundled with certain desktop GUIs and applications.
The hardcore linux enthusiasts will tell you to compile your own kernel. Have you ever tried it? It's not for beginners, and the choices you make during compile time would only serve to frustrate you later.
Look, I just want to use linux without needing to build it myself, but there are so many different ways to do things, there is no one way that would make everyone happy - no matter what changes you add on top of it.