The ISS isn't in a good orbit to service exploration spacecraft. The best inclination is about 28.5 degrees. Back in the 1980s when they were defining the space station specs, there were people like Buzz Aldrin pushing for the 28 degree inclination because it is better suited for lunar and interplanetary missions. However, once the decision was made to bring in the Russians, the orbit had to be about 51 degrees because that's the lowest inclination they can launch into from Balkinour (Tyuratam). You pay a substancial performance penalty for that inclination. If the goal is exploration, then the ISS isn't the way to go.

Reusability has proven to be less economical than hoped. For launches of people and payloads into orbit, the only reusable system to date has proven very expensive. Now, if you're talking about a reusable transit vehicle between the Earth and the moon or Mars, that's a different proposition. If you can keep the vehicle in orbit, service it for the next flight, then go, well, you're talking about a tremendous savings compared to launching a new throw-away transit vehicle for each flight.

NASA funding peaked around 1967, well before Nixon took office. At that point, NASA's budget was fully 5% of the federal budget. Adjusted for inflation, it was equal to about $30 billion in today's dollars. Nixon accelerated the damage that was already underway. When NASA was told that they weren't going to Mars, they tried to settle on a goal of a space station (beyond Skylab) serviced by a shuttle. They didn't even get the space station, so the Shuttle (now a capital "S") became their reason for being. Only, it was years late, grossly over budget, and never came near meeting the performance or serviceability goals. Even in the late 1970s, NASA was claiming a 2 week turnaround time. In reality, the turnaround time for a given orbiter was months.