IMHO, there were four cancellations that led us down the path we are currently on.
First was the cancellation of the X-15 and the subsequent follow-on. I personally believe that if we had pursued that to its ultimate end, we would have SSTO today.
Second was the cancellation of the NERVA. They were finally working the kinks out of it and poof, gone like the rest. Now we are renewing our interest in nuclear propulsion, however, 30 years have slipped by.
Third was the cancellation of the Saturn launch vehicle. Now we rely on Russian engines for one our heaviest lift vehicles. Even the Titan program is dead.
And finally the cancellation of the SP-100 space based nuclear power system and subsequent follow-on.
The emphasis, once again, is getting the cart-rider before the horse or the cart.
Just a few weeks ago, influentials (which I can't name, so you can take this for the anonymous and unaccountable observation it has to be) in the planning stages of the Mars venture are hopelessly focused on "what we'll do when our guy is on the planet." Hoo boy. There's this thing called "air" we need, first. Not to mention that awful deal-breaker "home again, home again, riggedy jig."
A long way from what we'll do when we get there.
They had a perfect satellite killer and/or reconnaissance bird with a little enhancement.
I still think NASA should invest more in basic research on new methods for propulsion - such as something similar to 'anti-gravity'.
We will never get very far using chemical rockets no matter how good or well developed. The distances are just too vast, and the power/fuel requirements just too great.