Not really, actually. If anything I'd be more worried about the more obscure stuff–very niche cards that people don't think to put up for sale. Although they are certainly harder to get than newer cards, cards from the best decks of the DPPT era are still very much available and not overly expensive, and those came out many years ago.
If you're worried about it for competitive reasons, I think it will stay relevant as long as it remains legal, and if it's relevant, someone is bound to put it up for sale at some point, unless it reaches Tropical Beach levels of rarity (which...I doubt). If you're worried for collecting reasons, the Expanded format is bound to rotate at some point, and an influx of older cards should flood the market soon after the announcement, increasing the supply substantially and therefore reducing the prize.
TL;DR: don't feel like you have to rush to buy older cards like that unless you have an event coming up. Even if the card gets scarce, the price won't go up that much unless it's something like a Charizard.