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

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.