See https://news.ycombinator.com/reply?id=21700802 and https://en.wikipedia.org/wiki/User:Guy_Macon/Wikipedia_has_C...
Re: "I have heard just the opposite: that having a decade-length endowment is generally a sign of hording by the non-profit, and unlikely to be contributing to the mission."
That's only true of charities where there is no natural limit on how much they can spend. If you are feeding orphans you can always do more until every orphan is fed. The WMF isn't like that, They have one job; keep Wikipedia and the sister projects like Wictionary online. That doesn't require ever-increasing spending other than to cover actual increases in hosting expenses and essential employees.
Imagine a future Wikipedia that is 100% funded by the endowment, which will always be there even if nobody donates, and which has no fundraising banners, just a small "donate" link. I think that is a goal worth pursuing.