The difference between different generations is wildly different and regulations aren't structured to allow for upgrading. It becomes a cost and regulatory burden thing - might as well rebuild then upgrade, very little to do with safety.

And I agree. I think this is a place where the regulations are broken. They should be changed to encourage new gen nuclear be built. Ideally, they could be tweaked so that the sites of old nuclear plants can be reused to produce new nuclear plants.