Former fan of Ubuntu
I'm still using it, actually, but without any enthusiasm. Actually, for serious work I'm using older versions, since the reliability of later versions declined so much, at least for the stuff I want to do. I still think Linux has potential, but Ubuntu is another example of a little engine that couldn't.
The big-donor financial model has two basic constraints. One is that the donor's pockets don't become empty, and it appears he's still got some change. However, the more serious constraint in Ubuntu's case is that the big donor doesn't start calling bad plays, and his calls started going bad a couple of years ago. Basically he listened to programmers who wanted to do flashier stuff and ignored users who wanted a practical alternative to Windows and Apple junk.
Maybe my taste is flawed, but I think Microsoft has awful products, and Apple has an awful anti-freedom philosophy. However their economic models work. Near as I can tell, the best Linux-related economic models are just barely managing to avoid collapsing, even after years of producing superior software with a superior philosophy.
My suggestion is for a small-donor-driven funding model. Imagine something like a stock market, but with charity shares recognizing sponsorship rather than for-profit ownership. Basic idea is the 'charity brokerage' helps with preparing the proposals (including planning for testing and evaluation of the results) and when enough donors like an idea, the money for that project starts flowing. (A viral version of it is described as 'reverse auction charity shares' somewhere on the web.) Basic idea is similar to Kickstarter and IndieGoGo, but with more support for project management...