We don't know the specific design constraints and requirements of the LS430 to know if cost cutting was a major factor as Toyota hasn't publicly released such information. Stainless steel exhausts are noisier than mild steel ones so that could have been a factor in making the LS430 more refined compared to the LS400. Also the replacement cost in the event of a rear end accident and therefore a factor was to reduce insurance premiums.
The LS400 was over engineered and therefore costs could be made whilst making the vehicle just as reliable for the first 10 years of its life, especially as the length of initial ownership has decreased over time. As it turns out there were a couple of design issues with the LS430 which contribute to a less reliable vehicle but that obviously isn't know at time of design.
Certainly Toyota today isn't all about cost saving; the current RX uses stainless manifolds and exhaust for weight saving and enhanced corrosion protection - they could have saved money by not doing that.
People's perception and threshold to detect and tolerate rattles, squeaks and other noises varies greatly. I've been in many vehicles where the owner swears their vehicle is rattle free, and many people of these forums claim the same, yet I hear multiple ones if the car goes over a pothole or you press trim panels etc.
In general over the 15 or so Toyota/Lexus vehicles I've owned the newer ones are much better than old for rattles, clicks, squeaks and other noises from the trim pieces moving, rubbing, flexing etc. My wife's UX is probably the best yet; some of the trim pieces may not be seen as ' high quality' but no real rattles or other noises yet. The only thing that lets it down are the very noisy 18" run flat tyres.