DS games simply won't function on other systems without major reworking or potentially awkward emulation. For the Nintendo DS, specifically, prices might get even worse than other systems, as the console employs a dual-screen format with touch support. As the DS's heyday recedes in the mirror, boxed cartridges will become more rare, and used ones will get broken or lost.
No one is mass-producing new games, much less hardware, and the backwards-compatible Nintendo 3DS was discontinued in September 2020. This happens to exemplify the main factor in rising game prices, both for the DS and retro game consoles, in general: scarcity. They don't even represent the most expensive, non-bundled item - that honor goes to a "preview trailer" for The Legend of Zelda: Twilight Princess, which has reportedly sold for $1,453 new. The above prices come from PriceCharting, a tracking and marketplace site. Related: Nintendo DS Hidden Gems & Underrated Games At the former two price points, it would be cheaper to buy a Switch or Switch Lite and play new game releases. Pokemon HeartGold is fetching up to $452 new, $215 complete, and $153 loose (cartridge only). But games like Castlevania: Dawn of Sorrow are worth $80+ new, and prices are getting out of control for even used copies of Pokemon games. Mario Kart DS can be purchased for as little as $10 without manuals and packaging. Some Nintendo DS games remain cheap, like Super Mario 64 DS or Mario Kart DS, which top out at about $32 and $29, respectively, on marketplaces like eBay at time of writing. The explanation is relatively simple, but it might still be enough to nudge retro gamers to look into other hobbies.
The Nintendo DS was one of the longest-lived gaming handhelds, with varying incarnations on sale from 2004 to 2013 - in the same timeframe, for reference, Microsoft launched both the Xbox 360 and Xbox One. Plenty of models can still be bought used, as can games, but the prices of some Nintendo DS games these days might trigger a little sticker shock.