You mention the ambiguity of the animating goals. Part of this is just linguistic: of course you want to push down these top-level goals to more granular, 0/1 goals ("was there an ancient lake at this site on Mars?" Answer: yes). That's what the NAS report does. Now that we know how abundant water was on ancient Mars, we'd like to return samples to see if there was life. That's the next couple of missions.
You raise a worthwhile point, I think, about immediately-graspable 0/1 goals, like "man on moon". I see your point - but I'm really only here to talk about the unmanned program because my limited expertise covers part of that area.
You like SpaceX. So do I - two good friends left my lab to work there. But their achievement ("revolutionized space") is not unique. The little Mars pathfinder had about that much history. There are plenty of other examples over the same time period, like exoplanet discovery (as you noted), cosmology, and a host of Earth science stuff.
