I'm currently writting a timeline in which the shuttle is cancelled late 1971.
Here's the basic scenario; the whole thing is much, much longer.
Big Gemini replace the shuttle, and fly on top a Titan III rocket.
Big Gemini is too little a program to fully replace the shuttle; while Nixon need to preserve aerospace jobs in California for the 1972 election.
So Nixon agree on a space station late 1972. This space station is built from Skylab experience, and the core is launched in 1979.
NASA works better than OTL because Congress pull strings efficiently, through the National Research Council.
A second shuttle program starts in the 80's, under Reagan's presidency, but ends five years later in X-33 fashion.
Result is that in 1989, when Bush starts its Space Exploration Initiative (SEI) planners are no longer bothered by Freedom, nor by the shuttle. Indeed with a space station in orbit from 1979, there's obviously no Freedom (nor ISS) from 1984.
So, with the shuttle dead, and the space station already in orbit, what will NASA do from 1990 to our days and beyond ?
Obvious!
Back to the Moon and Mars, if possible for cheap.
The answer is a small station at the Earth-Moon L2 libration point. From there, you can easily return to the Moon, then go to Mars using solar-electric propulsion.