It is just not true that twentieth-century technology made ornament more expensive: in fact, new methods of production made many kinds of ornament much cheaper than they had ever been. Absent changes in demand, technology would have changed the dominant methods and materials for producing ornament, and it would have had some effect on ornament’s design. But it would not have resulted in an overall decline. In fact, it would almost certainly have continued the nineteenth-century tendency toward the democratization of ornament, as it became affordable to a progressively wider market. Like furniture, clothes, pictures, shoes, holidays, carpets, and exotic fruit, ornament would have become abundantly available to ordinary people for the first time in history.
after reading this, my new personal pet theory is that one reason to use ornament is to make things look good when your construction tolerances aren't super tight - and that the rise of new technology has improved the ability to build things within those tight tolerances, and this allowed the rise of modernism and the decline of ornament.
this is pretty inspired by Christopher Alexander's A Pattern Language - he specifically advocates for adding trims both for reasons of aesthetics and because it makes things easier to build loosely. the point of a skirting board is to offer an opportunity for ornament, sure, but it also makes it much easier to neatly join a wall to a floor - leave a inch or two of messy plastering, quarter of an inch of expansion gaps, then cover them up with a bit of moulding.
