Some of the assertions here sound incorrect.
installing a new skin is complicated and requires shell access, which excludes / unnecessarily makes life harder for some of our re-users; and
Skins don't require shell access like upgrades used to. They only require filesystem access. Which is basically the same thing needed for extensions. This assertion looks wrong.
contributing a new skin is essentially impossible except for those of us in the ivory tower of the Wikimedia Foundation.
Modern was introduced by river. He's a root but he is not part of the WMF. Additionally our old standard default skin for almost a decade MonoBook, was created by Gabriel Wicke who back then was just a volunteer.
So this assertion is wrong too. The problem here is not the WMF. It's that to create a good core-level skin you need two completely different skill sets; The ability to design a really good and at the same time really functional design. And a complete understanding of MediaWiki that allows you to make designs that perfectly fit with MediaWiki's features rather than the features of other CMS systems. And sadly, finding someone with those kind of skill sets is very rare. So it's very rarely that a skin good enough to be considered for core shows up. Since being a core skin means that the skin should both be generic enough to be useful to every wiki and also make absolute perfect use of the different things we output, with no room for hacks.