Yes, and NASA found that their engines couldn't be rapidly reused during the early days of the Shuttle program. But the cost of the entirety of Shuttle refurbis

Yes, and NASA found that their engines couldn't be rapidly reused during the early days of the Shuttle program. But the cost of the entirety of Shuttle refurbishment was so large that the engines were just a small part of that. The point is, NASA flew the Shuttle and knew that it was a stretch to call it reusable, but they had hopes. SpaceX has a lot more data on their side and they know that there's nothing fundamentally not reusable about their stages. They've cycled the engines through enough simulated launches to know how long they'll last, and it's more than enough flights (dozens) to translate to massively reduced launch costs if they pull it off. That's just one reason why NASA's experience with the Shuttle is utterly irrelevant to the Falcon 9. Frankly, there's just not enough to do to a Falcon 9 to make it costly enough to reuse to end up with the problems of the Shuttle. It would have to be completely non-reusable for that to be an issue, and that's extremely unlikely given the design and the data we have so far (for example, the reuse of the sub-orbital testing vehicles in Texas).