Variety is good to a point. Too many alternatives and all you get is a bunch of under-resourced and unpolished results.
I disagree.
This assumes that progress on one distro doesn’t lead to progress on others.
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.
You think there is a dearth of software engineers out there who can’t spend time on something cool like a linux distro?
Well, yeah. Its pretty well established that there is fairly limited resources in open source. Loads of software engineers, very few contributors.