You raise a valid question: how many sources of funding does the Wikimedia Foundation need? The Bridgespan Group is a consultancy firm specialized in non-profits. They have been hired in the past by the Wikimedia Foundtion, for example in the period of strategy formation that led to the 2012-2015 Wikimedia strategy.
I do recall the Bridgespan Group analysis being shared on this list before when we've discussed fundraising and funding models (as evidence for why heavy reliance on the annual fundraiser was a good thing)
I am really really unsure about the conclusions of that report, for several reasons. Some of those reasons are quite dull and methodological (e.g. it is an ex post sample of post-1970 foundations that are now very successful, rather than an ex ante sample of charities employing different means and then examining what growth they end up with; or the arbitrary exclusion of universities and hospitals; or the fact the analysis only encompasses the USA; or the fact that the many "unknown"s and "none"s in the sample seem to get ignored in the analysis entirely.).
However my most important concern is that 73% of the "high growth" charities in the sample have a dominant income source of "government" or "service fees" (typically, from the government). That is to say, 73% of these high-growth charities achieved their high growth by delivering services the government wanted them to.
If you are a charity that finds its mission is completely aligned with delivering government programmes - great! Go for it. Get better and better at it and your organisation will grow, possibly really quickly.
If you are not in that position, then I really fail to see how this research applies to you. The Wikimedia movement definitely doesn't benefit from it.
Chris