The engineering doesn’t actually have to be better. It should be, to support that additional weight long term, but that doesn’t mean it has to be, or is. Take apart a new DC and a newer Plastic and the joints are identical. The diecast are heavier. That’s it. Take a plastic figure, a newer one, put some pot metal in the legs and it’s identical to a diecast. No new engineering.
And again...nothing against diecast as an idea. But it is the application that’s an issue. As well as how it’s touted as far superior. Even though it’s not in any way other then weight. For an extra hundred bucks a figure.