Some level of retroactive regulatory changes are required because the industry keeps discovering that the previous way of doing things was substantially less safe than previously assumed.
My favorite example of this is that during the golden era of cheap nuclear power mentioned in articles like this, it was the norm to run all the redundant control and monitoring wiring through the same narrow duct in a wall meant to stop fire spreading, fill it with highly flammable foam, and test the foam for air leaks using a bare candle flame. The way we learned this was a bad idea was because workers at Brown's Ferry Nuclear Power Plant actually managed to start a fire and take out a bunch of supposedly redundant monitoring and control systems whilst flooding the control room with smoke. This bad design made both the redundancy and the firestops that were meant to be there ineffective, and the stricter fire regulations required to prevent issues like this are a major cost.
You can't just assume that because something hasn't caused a major catastrophe yet that it's safe to continue doing either. This is such bad engineering practice and has played a role in so many major disasters across multiple industries there's even a specific name for it: the normalization of deviance. It's dangerous because it invalidates all the engineering and safety calculations that were meant to prevent disaster, replacing them with a gamble where no-one really knows the odds.
It is perhaps sometimes true that it is "less safe than previously assumed", but I'd guess that more often it's "we figured out a way to do it _even safer_", but in either one of these cases, 40+ year old nuclear tech was and is safer than coal power plants, which is the alternative. We crossed "safer than the alternatives" and "safe enough" decades ago. The safety regime in the US around nuclear is out of control and has no connection with any outside context.
Given when Brown's Ferry was built (construction started in 1966), plus just how many million American engineers, construction workers, and service members had hard-won WWII-era experience with "if you do it that way, then it may burn up / sink / explode with just one hit" design principals - one has to wonder at the management of the Brown's Ferry design process. How did they manage to keep all of the real grown-ups out of the room?
My favorite example of this is that during the golden era of cheap nuclear power mentioned in articles like this, it was the norm to run all the redundant control and monitoring wiring through the same narrow duct in a wall meant to stop fire spreading, fill it with highly flammable foam, and test the foam for air leaks using a bare candle flame. The way we learned this was a bad idea was because workers at Brown's Ferry Nuclear Power Plant actually managed to start a fire and take out a bunch of supposedly redundant monitoring and control systems whilst flooding the control room with smoke. This bad design made both the redundancy and the firestops that were meant to be there ineffective, and the stricter fire regulations required to prevent issues like this are a major cost.
You can't just assume that because something hasn't caused a major catastrophe yet that it's safe to continue doing either. This is such bad engineering practice and has played a role in so many major disasters across multiple industries there's even a specific name for it: the normalization of deviance. It's dangerous because it invalidates all the engineering and safety calculations that were meant to prevent disaster, replacing them with a gamble where no-one really knows the odds.