Thank you for the in depth reply! You make a very good point, and the timing of Apollo 13 with the budget decision is pretty damning, I'm convinced.
I will point out however that the budget was congressionally-mandated, and no funds were allocated for moon landings as they were in previous years; it would have been illegal to use funds dedicated to other areas for moon landings. Maybe I'm being overly pedantic here, but to say the 'funding was secured' as in the article implies the decision to cancel the remaining programs lay with NASA leadership; it would be more accurate to say that funding for the remaining programs, though possible, was not secured, most likely as an attempt to save face by congress/govt.
No, that's a great point. Let me rephrase it, they couldn't go to congress in 1970 and say, "hey, we've got $2.3B in other parts of NASA, here's what we're happy to cut so that we can keep Apollo."
Apollo 18, 19 and 20 were cancelled in 1970. 3+ years ahead of Apollo 18. Apollo 17 didn't happen until December 1972.
The US couldn't plug this funding "shortfall" in 3+ years out of the many, many parts of NASA?
It's pretty clear that the decision to kill Apollo had been made. The money is just how they chose to do it so that the POTUS didn't have to go on record cancelling Apollo. There was no room for negotiation. POTUS and Congress had decided that Apollo needed to die and so it died. How it died was relevant only so far as to serve as a mechanism to save face.
Yes, you're right. I just don't know how else to put it. The capital outlays for the components of the missions had already been committed to ahead of time. The physical capital was present; the main cost of the missions; those assets existed / were in place. I don't know what the right language is over here.