Exactly! When the (perceived) value of a LEGO set far exceeds the enjoyment someone has in actually building it, then the Lego stopped being a building-set and became an investment object. The purpose of an investment object is to keep it in pristine condition with the purpose of selling it in at a future date for an increased price.
When the Taj Mahal set was a rare set, people coveted to HAVE it as an investment object, believing it would increase in value over time. That made the demand for it.
Now that 10256 Taj Mahal is available for everyone, there aren't actually that many who genuinely want to build and display it.