Exactly, there's always a higher interest in the cards when some other aspect of the whole Pokemon industry is popular. When the Platinum game came out, for example, there was this renewed interest in the cards that's still going strong, plus you have the Arceus movie which added to people's interest too. Skyridge and Aquapolis suffered a lot, however, because they came out when there wasn't much going on with Pokemon in general.
And yes, there will always be a small society of hard-core Pokemon fans who hang onto it forever. In general, though, the demand for Pokemon cards will fluctuate until they become "vintage"; they'll reach that point where they are old enough that they can be collected solely based on the fact that they are old, and their value will only rise after that. That won't happen for a long time though, not until long after they've stopped being printed altogether.
