[WS:] Do you know that for a fact - and if so, how - or is it just your opinion about the state of knowledge pertaining to nuclear plant design?
And while you mentioned "consensus" there is nothing scientific about it - it is a social process involving egos, power relations, social connections, and back room dealing. It is rather common that what scientists or engineer recommend is not what is implemented. There are various reasons behind it. Some of if is cost, but most of it is egos, power relations, and dealing-and-wheeling. A may propose a brilliant plan, but if B dislikes A, he will counter his brilliant plan with his half baked ideas put hastily together just for the purpose of countering A. And if B can get in cahoots with C and D because they belong to the same country club, and can also get E onboard because he fucks his sister - guess which of these two proposals will be implemented.
And this is business as usual in many organizations, so it is not difficult to imagine that some variant of this process was in place in Fukushima.
Wojtek