The distributional impacts of major downturns and their effects on the groups donating to any particular charity aren't identical downturn to downtun; the fact that a particular major downturn didn't hurt WMF donations doesn't mean that would be true of the next one.
What historical downturn are you thinking of, and how much did the revenue of any charity lower purely as a result of that downturn (and not, e.g., because the charity's mission was no longer relevant)?
It sounds like some here are of the opinion that Wikipedia's revenue will never take a nosedive for any reason and that the bubble will never burst. Now where have I heard that kind of talk before?
Full disclosure: I am the author of the Wikipedia essay "Wikipedia has cancer"
What your describing is not just an economic downturn, because no historical downturn has ever come close to lowering donations sufficiently to starve Wikipedia. Rather, it's an end-of-the-world scenario, and Wikipedia's expenses would require a hell of a lot more than just paying AWS fees....
An extended economic downturn that dries up charitable donations.