To abandon Skylab, and Apollo as well, was a real waste in my humble opinion.
I think that Skylab was a real milestone after the Apollo lunar landings. It's interior size was amazing and it offered the opportunity to do research for manned Mars missions later on. I think it is possible, that if it would have kept in operation, while NASA would have developed the Saturn launch vehicle family and Apollo spacercraft further, we already could be on the way to Mars manned (by around 2000, regarding the vision of Wernher von Braun just before he left NASA disappointed after Apollo was canceled). Related to manned Mars and further manned Moon missions, I think the way even Michael Griffin obviously thinks too, that the Space Shuttle was not the very best choice, although it still is the biggest milestone for now. But I think it is time to go further finally.