It's not an issue of "WMF can't hire enough designers" or "WMF can't hire good designers".
I worked for WMF in a design-adjacent role for the better part of a decade. WMF has excellent designers, and in sufficient numbers to build a modern user interface on desktop--one that looks modern and also prioritizes the needs of Wikipedia's readers (editors can always load up an old skin if they don't like the new one).
The mobile site and Wikipedia apps have a much more modern look-and-feel and are clearly focused on making Wikipedia "work" for its largest set of users: readers. If the desktop site lags on the design side, that may be because when WMF has tried to make UI changes to the desktop site in the past, or even just proposed them, they've received loud and angry push back from members of a second (smaller, but equally important) set of users: editors.
WMF, understandably, tries to avoid angering editors (believe it or not).
At the software company I work for now, if we make a change that annoys our users--pretty much all of whom are "power users" with needs every bit as complex and idiosyncratic as your average Admin--we hear about it. But no one threatens to disable that change across the platform. And it's relatively rare for a user to accuse us of being stupid or lazy or malicious--at least, its rare on for that to happen on public mailing lists or in our own forums.
That doesn't mean the stakes are any lower: if we make the software worse, we probably lose customers. But we have the autonomy to make the changes in the first place, see what happens, and then build from there or fix our mistakes or even roll things back if we need to.
WMF product teams work in an environment where their competence and good faith are frequently, and publicly, called into question. An environment where one set of end users (editors) has a great deal of both soft and hard power to block changes, even when those changes are intended for--and indeed, primarily affect--a different set of end users (readers).
Speaking as someone who worked inside of that environment, I can say that it can feel like even targeted, clearly motivated and well-justified changes aimed at improving the reader experience aren't worth the cost.
There are plenty of other factors at play, but I'm sure I've already said enough to anger plenty of you, so I'll leave it there.
I no longer work for WMF and my opinions are my own.