Exclusivity plays no role in rarity, and I don't think it plays a huge role in value either. Anybody who wants to play Stadium Events can play it in the form of World Class Track Meet. The fact that the game is common and cheap in another form doesn't stop Stadium Events from pulling in massive figures. When valuable games are ported/remade, sometimes the value takes some degree of a hit, but it usually doesn't make a big difference.
While that was a concern immediately following the crash in the US, I think video games have been around for too long to ultimately be a fad. It's a basic form of entertainment now, like movies and TV, and will always have collectors among the fans. Prices will fluctuate as waves of nostalgia come and go, just as prices dropped on pre-crash games, but even past its height of collectibility, there are still people collecting pre-crash games and paying good money for the rarer games.