<Anyway, my point is that with each accident, a flaw is discovered and corrected so that it doesn’t happen again. NASA decreases the odds of another accident. Over time the shuttle become safer.

Yes, we can fix flaws as we find them, hopefully without losing all the Shuttles and crews in the process. However, over time, the Shuttle also becomes older and harder to maintain. It doesn't necessarily become safer. I think it was Columbia that had electrical hickups during launch a few years ago due to damaged wiring. The point is that improved safety isn't a given with increased age.

They fixed the O-ring problem following Challenger. Fixing the foam shedding ET problem might prove more difficult. The key vulnerability will still be the delicate tiles. I doubt NASA is going to replace them anytime soon. With a vehicle as complex as the Shuttle and with as many single point failures as there are in the design, the chances of another accident will always be there.