This is strange logic. You're saying that they shouldn't fire the cop because any new hires are going to be incompetent? This doesn't make sense. You can both fire the cop and call for better training. Besides, if we're constantly firing cops for not following basic gun safety, either we solve the problem or it becomes cost prohibitive to not properly train. You can indeed have both.

One, I think this cop will argue he was following basic gun safety, that he was authorized to use deadly force against anyone or anything in the house. I don't think this cop was incompetent nor would his replacements be; I think he was doing what he was trained to do, and they'll train his replacements the same. You've got to make his actions fireable, first. You can't just wish they were.

Second, we won't fire the cops who don't follow basic gun safety - we will at most fire the ones who get caught on video as part of a story of national importance. This cop isn't the one bad-trigger-discipline cop in that department, let alone the country. Fire him without figuring out how to systemically fire them all, and cops will dismiss it as politically-motivated persecution, have some protests about #BackTheBlue or maybe even a work stoppage, and then go back to business as usual, with the same trigger discipline.

And the fired cop will get a job in another department.

I agree you can have both. I just think you can't have one without the other.

The gun safety rule applies to things you actively intend to destroy, not things you're merely "authorized" to destroy. Furthermore, cops aren't authorized to use deadly force against anyone in the house a priori, only if there's sufficient danger (IANAL).

(Not disagreeing with your overall point, just saying that it would be (IMO) laughable for a cop to try to make those arguments, even considering their training.)