Engineering difficulty is often put forward as a limiting factor. But actually I think history suggests that with enough resources we can accomplish things. And in fact it is possible for a project to have astonishingly brilliant engineering and still be disappointing in terms of wider impact (see the space shuttle).

The limiting factor is much more mundane. Getting humans to collaborate selflessly on complex projects is difficult. Effective communication in large groups is a massive unsolved problem. That is what holds us back, not engineering complexity. Perhaps the true genius of SpaceX is using Mars to bring people together. Something the bean counters at Boeing never thought of!