If engagement rings were as ubiquitous and easy to generate as Ghibli images have become, they would lose their value very quickly -- not even just in the monetary sense, but the sentimental value across the market would crash for this particular trinket. It wouldn't be about picking the right one anymore, it would be finding some other thing that better conveys status or love through scarcity.
If you have a 3d printer you'd know this feeling where abundance diminishes the value of something directly. Any pure plastic items you have are reduced to junk very quickly once you know you can basically have anything on a whim (exceptions for things with utility, however these are still printable). If I could print 30 rings a day, my partner wouldn't want any of them as a show of my undying love. Something more special and rare and thoughtful would have to take its place.
This isn't meant to come across as shallow in any way, its just classic supply and demand relating to non monetary value.