What's The Logic Used In Key Dates Getting Less Valuable Over Time?

Price adjustments aren't necessarily a election of lack of demand. Over the last 10 years the internet market has absolutely exploded and phones basically turned into laptops that even 10 years ago we were no where close to what we were five years ago or today.

Not all keys are equal, some were keys because of the historic allure of them, some are grade rarity keys, others are actually keys. The internet market and the accessibility of both has pulled back the curtain on what is actually available.

Take the S VDB that you mentioned. 40 years ago, or 30 years ago, really even 20 years ago that was a magically coin. People were sold on it being the key date, something very special that is hard to find. People were basically at the mercy of what their LCS or a local show had. Then along comes the internet and now if you had the money with the possible exception of the very top grade you can basically build a grading set of them in 20 minutes online.

ebay alone has a couple hundred S VDBs at any given time. The new availability of the internet market and exposure of what is actually out there has caused prices to adjust on things once thought to be hard to find.

No longer is someone dependent on the local supply as basically the whole supply chain is opened up to them now. Diamonds aren't rare at all, but their supply chain is controlled just like the coin supply chain used to be. Now that the control is gone prices are/have adjusted as collectors and the market have reacted to the new open supply visibility.

Could it go back up sure, could it go down more sure,
but when comparing pre and post internet marketing pricing a lot more is in play than simple demand

Prices go up, prices go down.
I only keep a very loose check on price trends.
Why?
Overall, over a 50 year time period, the price trends of collectible coins is about 3% per year, compounded. Only a bit less than for stocks and bonds.

Be a lifetime collector - less hassles! What then does a long term collector care about the price ?
A dealer may worry - he has to make a profit on short term turnover (cash flow).

I don't think there are any guarantees that key date coins or any other coins for that matter will hold their value over time. They might, but if people lose interest in coin collecting (which seems possible given that cash and change is declining in usage), collectible coins will fall in value. Too many sets being passed along to heirs that have no interest will lead to excessive supply with lower demand. That means lower prices typically.

Very interesting topic -thanks.

You can go on the PCGS site and look at the price history over time since 2004. A lot of prices today are very similar to what they were back then. So yes, the key dates will hold their value, but actually decrease in value after inflation.

A lot of collector interest in assembling complete date sets has waned, so many key dates have lost premiums. There are some that have always held strong. The 1916-D Mercury dime in AG-03 has always been popular and people look for that grade. It gets too expensive in higher grades so they look for the $300 coin they can sell for $350.

Pretty much. I am starting to look like most of the people I have seen at the shows all my life.

In truth, I feel like most US types are a bubble that has been ready to pop for decades. I understand the appeal of collecting from your own country, but the prices that ordinary 19th century US coins get is absurd!

I nearly completed my Japanese type set (Tokugawa and modern, no gold, no Trade dollar, no 1870 yen) in mostly XF/AU for about $1k, and that includes a few key dates. 150-175 coins in total; can't remember exactly.

I've nearly completed my ancient Roman set (Augustus through Valentinian III in the west, Justinian in the east) with 96 bona fide emperors, or 144 people (including wives, children that died before taking power) and spent less than most people do on a complete 7070 with gold. (And that's with about 50% in silver, nearly all in high grade. Two of my coins are unique; several are known from only a few specimens; fifteen of the emperors I own were in power for only a few months and only minted a few thousand coins total)

My interest in US classics lasted only as long as it took me to figure out what else was out there, and what gave the best bang for the buck. I've noticed that of the YN who grow up to become more serious collectors in young adulthood, many of us are drifting away from overpriced bust coinage and grade-fetishism and investing ourselves and our money into cheaper world coins and ancients. I might get into US coins eventually, but only after the bubble bursts and we see early US coinage fall into line with their worldwide contemporaries.