But you also have to remember that the poor flying of the Hoatzin is probably not an ancestral condition. There’s no reason to think that the ancestors of the Hoatzin weren’t just as good at flying as any other bird, but the Hoatzin has evolved poorer flight, just like the chicken or turkey, or the penguin or ostrich or kiwi for that matter. We know that there were fully flying birds back in the Cretaceous, so the Cretaceous ancestors of modern flightless or poorly-flying birds were almost certainly fliers. After all, the ground-dwelling niches flightless birds have today were mostly occupied by non-avian dinosaurs. There’s no niche for a Struthio when Struthiomimus is still running around.
I think the main support of the “ground up” origin is the seeming lack of arboreal ancestral dinosaurs for “tree down” flight to have evolved in. Unlike mammals, which have numerous arboreal lines, dinosaurs seem to have been pretty much all ground dwellers.
Well I wasn’t being entirely serious. It just *looks * like it hasn’t bothered to evolve since its dinosaur cousins went extinct 
I thought the Hoatzin was a good example for the OP since you could imagine at it as what a “partially” evolved***** bird might look like, and how even a little bit of ability in the flying department can be an advantage.
*****Yes I know that’s nonsense, I guess the only fully evolved organisms are ones that are extinct.
Many paleontologists are now leaning toward the “trees-down” origin. Most of the feathered dinosaur fossils coming out of China represent very small tree-dwelling species, with wings suitable for gliding or parachuting to the ground (as in Sinornithosaurus ). Some species like Microraptor and Cryptovolans even had an extra set of wings ontheir feet that may have made running on the ground awkward.
