Who's going to fund the upfront development? "We're going to make a new desktop OS that takes on both Apple and Microsoft" sounds like a hard sell for the VC crowd.
And that's the hard part....OS/2, NEXTSTEP/x86, and BeOS were examples of alternative OSes that failed to successfully take on Microsoft in the 1990s. It would be difficult for a VC firm to fund a business model that has failed multiple times in the 1990s. Moreover, the desktop market as a whole is shrinking, and my understanding is VC firms are interested in growing markets. An alternative OS dedicated to power users is definitely a niche market.
I wonder, though, if these is enough interest in the FOSS community to make such an idea a reality, where the project could be started by volunteers and donations could be requested?
It could be, but it seems like that would depend on the brand awareness / credentials of the team writing the software. Note that KDE, Gnome, etc. have been trying to do exactly that for decades, with only marginal success.
So if you got the team that wrote the Apple Aqua UI on board to write it, and have great PR, then maybe. If you get the KDE or Gnome team, no way.
Who's going to fund the upfront development? "We're going to make a new desktop OS that takes on both Apple and Microsoft" sounds like a hard sell for the VC crowd.