"Re-using Shuttle engine and solid booster..."
Sure, NASA is scraping any 40-50 years old technology because it looks its engineers are utterly incapable of developing and delivering anything newer and better. They had to shelve the Shuttle as well because they were no longer able to fly it safely - for lack of competences.
NASA has to be fully rebooted. It was created when the military was trying to make rockets and made only disasters, but now NASA it's at that same point - because it has now exactly the same arrogant, stubborn bureaucratic and incompetent structure.