Then the revolutionary advances that require heavy expenditures just won’t happen. Would private enterprise have landed anyone on the Moon if there were a bunch of MBA’s doing discounted cash flow analysis and passing judgment on any such proposal? Would there be a Panama Canal or a transcontinental railroad without government funding and organization? Would there even have been an atomic bomb? Gotta be realistic here - large projects require a group effort without a need for a near-term low-risk positive cash flow requirement, and that means using the government to do it. The privatization ideology applied inappropriately or at the wrong time has not only been stultifying, it has been destructive - witness the space station languor or the Columbia crash, for instance.
This is a good OP, and I’d like to define it as something that not only benefits all humanity tangibly, directly and through spinoffs, but inspires us spiritually and for a long period, too. The science has to be reachable enough to make it an engineering project. Ending world hunger and disease might do the trick, but I agree we need something big and visible to take pictures of. I’d like to see something other than space missions in the mix, but frankly I don’t have any good ideas other than that.
Looking at big projects of the past, they’ve all had a charismatic visionary to make them happen, or at least get them going, within both the government and the private sector. There probably wouldn’t have been a Suez or Panama Canal without De Lesseps, or a man-in-space program without Khrushchev and Kennedy. But, given our current cynical and government-hostile political climate (at least in the US), it’s almost impossible for any such personality to get anywhere.