Perhaps a more instructive version of your metaphor might be a fire extinguisher which emits a flame retardant for the first second which explodes when it contacts a certain chemical. Would it be better if it didn’t do that? Sure. But the flame retardant which doesn’t explode is expensive and changing the design of the fire extinguisher is even more expensive. Besides, the fire extinguisher would still work fine, you just have to not use it on that one particular type of chemical fire.

As others have covered in a lot more detail, the operators of reactor four had to take a number of extraordinary measures to put the reactor into a state where the graphite tipped control rods could cause a catastrophic failure. Worse, they didn’t even know there was a catastrophic failure state they needed to look out for. To go back to the fire extinguisher metaphor, no one ever warned them about the chemical.

That’s supposed to be the big revelation at the end of the show. Legasov knew about the potential for an RBMK reactor to explode, but the people running Chernobyl that night didn’t. Even though the operators where running the reactor in reckless and dangerous manner, they only thought they were risking a shutdown. They didn’t know they were courting utter disaster because they had never been told that was a possibility.