The problem is that no one bothered to check whether the software worked when they removed the hardware. It's not a good thing that there was a bug in the first

The problem is that no one bothered to check whether the software worked when they removed the hardware. It's not a good thing that there was a bug in the first place, but proper procedures could easily have caught it before people were killed. That's why you have multiple systems to prevent catastrophic failures. What makes the Therac-25 notable is the novelty of a software problem killing people. If it had been a second hardware safety mechanism that had been removed, it wouldn't be as notorious - but the engineering failure would be equivalent.