If the shuttle program is canceled, the space program will die. Human exploration fires the imagination of the citizenry. Robots and other automated probes will not generate support and mass politics will win out. The funds (and tolerance for risk) needed for a program to Mars or back to the moon are beyond the private sector. This is truly a role for big G government. Without the vision, excitement and potential of manned exploration and colonies, the US will only have a LEO capability for satellite launches and other near earth uses. Manned space flight and shuttle type vehicles may not be the most cost effective, but their absence would restrict space activities to narrow, short-term, projects only IMHO.
Now is the time to brace up our political leaders to support not only a return of the shuttle fleet to service ASAP, but also the development of a new generation spacecraft to replace the shuttles as a national priority. NASA has tried and failed to develop such a replacement several times, but a combination of techical difficulties and lack of funding has doomed each such effort. Now is the time to decide that we are serious about our space capabilities and put sufficient effort and funding into the program to succeed.
I hate to burst your bubble--the space shuttle is not a space exploration vehicle--it is a badly designed cargo deployment ship with passenger room. NASA has zero manned space exploration vehicles.
>>If the shuttle program is canceled, the space program will die<<
Then the space program (the American one) is dead, because the shuttle will never fly again.