I have been a Wikipedian with a registered account for a few years, and I have reflected on these issues. The article asks, "But what if the decline in engagement has little to do with culture or the design of the site? What if, instead, it's that there's just less for new Wikipedians to do?"

It's the culture. There is plenty to do. Indeed, every time I read a Wikipedia article, I find something in every article that turns on my inner editor mode. But my log-in information is only stored on my office computer, not on my wife's computer or my children's computer, so when I am reading along in Wikipedia with my family and I see a mistake, I usually grumble rather than Wikignome and fix the problem.

http://en.wikipedia.org/wiki/Wikipedia:SOFIXIT

I do Wikignome regularly when I'm reading Wikipedia for one reason or another on my office computer. That gives me solely the satisfaction of fixing a problem, and no satisfaction at all of supposing that that builds up my reputation among other Wikipedians or that the fixes will even persist in further updates to the pages.

Quite apart from the articles that have jump-right-out-at-you errors, convenient to fix for anyone who knows grammar, spelling, or basic facts of the world, there are many, many articles that are readily apparent examples of factual mistakes from more subtle causes such as edit wars

http://en.wikipedia.org/wiki/Wikipedia:Lamest_edit_wars

that often persist even after action of the Arbitration Committee.

http://en.wikipedia.org/wiki/Wikipedia:Arbitration/Requests/...

A user who is knowledgeable about SOURCES and who has professional editing experience and academic editing experience (as I have) is hardly likely to find Wikipedia welcoming. Wikipedia's culture was set in the days when it was the "encyclopedia anyone can edit," that is, the encyclopedia where anyone can make something up, and then as Wikipedia grew an administrative apparatus, administering was used more for turf battles and point-of-view pushing than it was used for editing to encyclopedic quality or fact-checking.

I think a philanthropist with a budget the size of Wikipedia's budget over a few years (a large amount of money, but pocket change for a billionaire) could build a Wikipedia competitor that could do quite a lot better. Maybe no one will ever think that that is worthwhile. For sure, many people who are well aware of how incomplete Wikipedia still is regret the off-putting culture of Wikipedia, and will be dissuaded from spending much volunteer time to improve it.

AFTER EDIT: A grandchild comment noted that "One perk of competing with Wikipedia is that you can use Wikipedia articles as a base." And indeed, if a philanthropist's goal for a competing project were improving the quality of online encyclopedias, licensing the competing project's articles in the same way Wikipedia's are licensed would mean that good articles could work their way back into Wikipedia--a very disruptive strategy indeed for improving content.