Some of these estimates from 2001 to today are very iffy. For example, one Chinese artwork from the 8th century was depreciated over the course of 15 years, but the Chinese market is strong today. There's no way something that is Chinese would go down in value. Especially artwork..
shareDon't know about that specific case, but in general, yes something could depreciate for a variety of reasons.
The most glaring example is sports cards. There was a boom in the late 1980s to mid 1990s. Then many companies jumped on the bandwagon and flooded the market, causing a collapse in prices, though early cards retain their value.
Fads and crazes. Think beanie babies and the like. The fad or craze peters out.
Item not as rare as originally thought. Say some item out of the blue fetches a high price. Now antique dealers and others start searching for the same or similar item and examples turn up.
Collector interest. This can rise and fall.
I'm sure that there are other reasons as well.
Of course things could depreciate, an erratic economy is part of the reason but also fifteen years is pretty much a generation of people now gone (certainly many of the elderly guests in these 90's and early 21st century episodes are no longer with us) and a younger one not necessarily interested in particular items. Add to it many of these "appraisers" are very generous with their estimated values and a more level-headed and perhaps experience team 15 years later can really look into what these things are worth and not just give a figure off the top of their head or a fast internet search.
shareI think that in some of these cases a lot of the value is based on the ephemeral demand for these things. Some stuff will always have a certain level of demand, but other things might see fads come and go for them.
Unless Alpert's covered in bacon grease, I don't think Hugo can track anything.
Another thing is that the show itself can result in more examples of similar/identical antiques being put on the market as their owners realize they've got something worth money, leading to market saturation and price declines.
That Tang dynasty thing not going up in value was weird, though. Can't explain that aside from guessing that it was a ridiculously high estimate in the first place that was just wrong for its time.
Death to shakeycam directors!