What's Going On With Sports Card Values?

Why do cards go down in value? Supply and demand are supposed to be the reason. There should never be a reason for any vintage cards (1969 & older) to go down in value. They only keep getting older! Beckett price guide states that their regional coorespondents report back to them with card market activity. I am personally out at card shows on a regular basis for the past 22 years. I have spoken to 100's of dealers and not 1 of them have told me that a Beckett representative approached them regarding their card sales. What does that mean? Is it possible that the 1000+ card shows that I have attended over the years have been missed by the price guide experts? Come on! Every once in a while I glance in the price guide to check values of cards that I have only to see that some of the major cards that I own have gone down in value. Why have some of these player's cards gone down in value: Emmitt Smith, Dan Marino, John Elway, Mickey Mantle, Michael Jordan (Rookie), Willie Mays (1953), Wayne Gretzky (Rookie), Tom Seaver (Rookie), etc. You get the idea. Collectors want a good deal. They want to pay less than book price for a card. When they do, does that mean that the card now has gone down in value? Will the price guide reflect this new purchase as the new value? Will collectors now try to buy this card at a new discounted price once the price has been lowered because of that purchase? Will the price guide now reflect this new purchase and lower the value again? This can go on and on until the card has absolutely no value. This is the reason that the hobby will not last longer than the present generation. People cannot afford the new cards and when they do purchase them, they lose value. When collectors invest in vintage, some of these cards go down in value. This puts a bad taste in everyone's mouth. The kids nowadays do not collect sports cards. There is no incentive to. Why should they lose money that they can put towards new phones and video games.
I understand that newer cards fluctuate in value depending on player's performances, actions, etc. But once a card has aged and the player depicted on the card has established themself as a star, their card should only go up in value as it gets older...like an antique. What are your thoughts? By the way, you can purchase 1000's of cards and collectibles on our website. Continue to support our hobby!!