I think that with any well-established collectibles market, there will be natural ebbs and flows in interest over an extended time. Was baseball huge in 1890? Not tremendously, but there were still lots of people who collected the cards. Then there was probably something of a lull in the hobby for 20 years as there was little supply of new material. But then a new generation of collectors emerged with the next "golden age" of collecting. Various factors will always divert attention...wars, depressions, changing tastes and technology, etc. But the card collecting market overall has weathered these changes for almost a century and a half and is stronger than ever.
I think that the popularity of baseball as a sport will not have more than a negligible effect on the card market. I am no die-hard baseball fan. I am 36 years old. I live in a major league city (well, technically...I'm a Royals fan
) and have attended maybe 10-15 games in the past 20 years. But I have been a passionate card collector for my entire life and imagine that I will continue to be. Of course, that's not to say that there will not be another 10, 15, 20 year lull in collecting as tastes change, but why would anybody be certain that tastes would not shift again in a decade or two and baseball would be right back in the forefront? Or the next cycle of card collecting popularity is born for whatever reason? I don't think that anything special happened in 1997 (or in 1989 or 1981 or 1952 or 1909) that caused a massive increase in interest in the game of baseball, but new and interesting card releases got many (kids AND adults) into the hobby. I see no reason why such a thing could not happen again and the cycle will continue as it has for generations.