What underpins the broad shift in (broadly Western) design from highly decorated things in times up to the 19th century to the eschewal of decoration later in the 20th century and beyond? Here is a rough-cut of an economic theory of decoration.
Prior to the industrial revolution, individual things were expensive. The cost of making the stuff was in material—not necessarily raw material, but getting material from its raw state to a state where it can be used. A lot of this is semi-skilled labour cost, but a lot of it. There is an interesting argument that a shirt in mediaeval times cost around 3500 USD in modern money. For example, spinning (by hand) the thread costs 500 hours of work, and weaving the cloth another 72. Therefore, each shirt was a very valued object, and worn to exhaustion, frequently repaired, and repurposed if it was not viable in its original form (there is a nice discussion of this in Susannah Walker’s recent book The Life of Stuff).
Similarly for other material. Transport costs in an era where horse and human motive power was the prime driving force was huge. The cost of getting material to a building site—a minor cost of building a modern building—might have been a huge proportion of the cost.
By contrast, the marginal cost of adding some decorative addition to something is therefore small. If you have paid hundreds of hours of labour to get your basic shirt, adding a few more days to add some decoration is a minor marginal cost.
By contrast, with 20th century manufacturing techniques, the cost of producing the object is much less: the core materials can be produced and shipped at low cost, and a lot of the cost is coordinating the various low-cost steps and delivering the object to the final consumer. The relative labour cost of adding elaborate decoration is high. This doesn’t fully stand up—after all, modern techniques of manufacture can add some decoration very cheaply and easily. But perhaps in some cases it holds—I can see this particularly in the case of architecture, where the logistical cost of coordinating lots of special decorative components will be high.