You're right there's a reason Apollo 18 was cancelled, but I'm not sure what you'd reference this. The main issue is that Nixon was increasingly paranoid that there was going to be a catastrophic failure in the Apollo program, and that it would affect his political career. He tried to hard to the cancel the program immediately after the first successful landing. NASA had already drawn up plans for not only getting to Mars, but for a complete human settlement of outer space with large space stations and more. This was all cancelled.

So I think the comparison with flight is perfect. Imagine after the Wright Bros. flight, which countless people had died in the process of seeking to achieve, we had one uninvolved entity able to say 'yeah, I'm responsible for all this' and then canceled everything in a simple self-motivated political calculation to try to 'go out on top' so to speak.

The Moon was never the goal, anymore than the Wright Flyer was the goal. It was one small step on a very lengthy journey - a major milestone for sure, but nowhere near the end of the road. And that's where we remain in space, but thanks to the fact that so much expertise was lost in the ~60 year do-nothing era, we're now having to essentially start over. On the bright side, this time the driver's going to be private industry, just like with airplanes - and there will be no myopic politician to cancel it.