I've long thought that our emphasis on reusable launchers was misplaced, though I think SpaceX has as good a chance of anybody of finally making that idea economically practical. Much of the cost of an Apollo mission came from throwing away a perfectly good LM after each landing just because the tanks were empty. We need to figure out how to refuel these spacecraft with propellants made somewhere other than the bottom of the earth's gravity well.
The engine bell had an ablative surface instead of being actively cooled by the fuel. So the engine would need to be changed, as well as allowing refuelling. And there's the rub...doing all of that increased the weight, which increases the fuel load needed to get it off the ground in the first place. Which means that you need a bigger booster....
Expensive as it was, it was probably cheaper to implement a "throw-away" policy. Certainly to hit Kennedy's timescale the designers had no other choice. It had to be as simple and as light as possible otherwise they'd never have done in within the decade.
Fast. Cheap. Safe. Pick any two, but you can't have all three. A fast, cheap program won't be safe. A safe, cheap program won't be cheap. And so on.
