I wouldn't think of old American stuff as overbuilt. That trait is found in many countries engineering of that time period. I think when it happened too, it wasn't the intent. I've heard of some manufacturers doing it (as in, "what does it need? lets make it a little more than that."), but I think it was more a byproduct of the way things were built than an intentional design trait.
If anything I think modern stuff is "over designed," as in, the engineers over-analyze materials and designs and build them to the minimum specs necessary. The problem with that is that the computer doesn't always do the best job of predicting the future or leaving enough room for error. The old way of doing it did involve a lot of analysis and calculation, but IMO it also involved a LOT more field experience. When someone made a machine back them, they would first look at what was already done, copy it, and improve it from there. There was a lot more physical trial and error involved. When someone makes a machine now, the process is entirely digital until some kind of "beta testing" determines if the machine will fit the order. Often by that point the wheels of production are already turning.
IMO old American built stuff exhibited a good deal of quality, even the "cheap" stuff, but if there was anything that made them stand out then and even now, I think it's market driven innovation. That trait is also found in many other countries, but I think American's been doing it the longest (IMHO). A majority of products are conceived because some Joe saw a need, mixed it with some bright ideas, and sold it in a way that would reach as many of the needs as possible. It's still done that way today, it's just that the market actually wants disposable products.