There is a difference between feature development and distro maintenance/packaging.
Feature development is done upstream and does flow down to others.
Distro maintenance and packaging is downstream, and almost never provides value to other distros. It usually doesn't even provide value to the next release. Distro maintenance is a hard, thankless Sisyphean task.
We can try to lower the barrier of entry. But nowadays open source maintainers have to actually limit controbutions due to a significant increase in supply chain attacks and generally untrustworthy code contributions.
I tend to agree. I like being able to install whatever distro I want and add the DE of my choice, and there is a glut of different combos to choose from.
However, are KDE and Gnome going to gradually focus on making their respective DEs work on their own branded OS, rather than any old base system? I know that's a worst case scenario, but putting a lot of added effort into a full OS is a nontrivial investment for a desktop environment. Some mission drift might be expected.