On the one hand, it satisfies the goal of less duplication.

On the other hand, it makes each downstream distribution as volatile as the sum of all upstream volatility.

On the other other hand, that would make it harder to have a meaningful certification process.

Finally, it would make the installation process harder, which is one area that needs fewer problems, not more. I think as a practical matter major downstream distros are going to have to provide the whole shebang.