Already from the start, the word "foundation" in the title of the Wikimedia Foundation has caused confusion. In Florida, you register a corporation, and "foundation" is just part of a name. In some countries in Europe, there are completely different laws for corporations, associations and foundations (German: Stiftung).
In short, a foundation (Stiftung) is an immutable long-term, self-governing holder of money. A typically example is the Nobel Foundation, which holds the money inherited from Alfred Nobel, and every year spends the interest on the Nobel Prizes.
Apparently, the WMF has a problem to foresee how much each year's donation campaign will bring in, and how the coming year's budget can be made to fit this. Perhaps each year will raise less and less money, and that we already have the best years behind us. Is there a way we could reach better long-term stability? Should the WMF set up a long-term fund and move some of this year's money there, as a reserve for future meager years? If the interest rate is 4% then a fund which is 25 times bigger than the budget can support it in whole for ever. But even a smaller fund might be a good help. Should donors be given the option of giving to the current budget or giving to the fund? Has this been discussed?
(Some people would claim they can easily earn more than 4% annual interest. Obviously, they should start savings banks.)