The U.S. is not really serious about putting men (and women) into space. The U.S. is decadent, and more concerned with empire on earth and supporting its welfare state clientele so as to keep the current Democrat-Republican cabal in power. There is no vision other than the latest cell-phone technology.
If other countries do not take the lead, the human race will stagnate and die in its own pool of fecal matter.
Hopefully, some other country will assume leadership -- China, Japan, India, Russia... ??
There have been a few attempts to build successors to the shuttle, but they were shut down as soon as they ran into problems. E.g., the X-33.