Nixon cancelled them because he was increasingly worried about a catastrophic accident and felt it could damage his political career and ambitions. Seriously. [1] If he only knew... Outside of that you're correct. We were already set for progress onto Mars. In a parallel timeline where one man's political ambitions, and myopia, didn't override the interests of an entire species - we could already viably have multiple parallel civilizations scattered throughout the solar system, and perhaps already be flirting with abstract concepts of moving beyond just the solar system. We're left with our timeline, but hopefully people 60 years from now might look back at us more fondly for our actions of today, and tomorrow.

[1] - https://www.planetary.org/articles/1027-when-nixon-stopped-h...