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

It was a sales pitch trying to get the most money from you. The highest quality coins are more expensive, if you are getting lower grades the key dates are more expensive.

It's like the people trying to sell you precious metals who say they will only go up in value, if they actually believed that they would be buying all they could not selling.

Low grade keys have a more limited market than a lot of coins, they lack the eye appeal of the expensive coins but are much more expensive than the common dates in the same grades. Some people will love them as that's all they can afford and there is nothing wrong with that, but a lot of people would rather have an AU or higher more common date than a G key.

Series get hot and cold, the market gets hot and cold, certain things within the market get hot and cold. What is going up in value today may be going down next year and vice versa, its always a moving target

What are you referring to as a key date? A low grade 1931-s Lincoln is a lot different than a low grade 1916 Standing Liberty quarter. Can't respond to your question in generalities.

Good read though. I'm interested to see where this goes.

I ignore ALL advertising irrespective of it's type or character, unless I know that it will be of specific advantage to me.
That includes verbal sales pitches.

My approach to buying is to always try to get he best value for money, with regard to condition, in ANY condition range.

Makes cherry picking easier.

Maybe you misunderstood. The price for this key date has fallen considerably since 2010, making it a terrible investment.

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.