Usually they hit a plateau and then decline a bit. For instance there was a while where a CIB original NES Zelda cartridge would go well over $100, but now they're usually closer to $75 at tops. SNES ones had a similar price structure. N64 still usually is around $100 or so CIB (not sealed). The DSlite though (more relative to your question) can be found cheaper than MSRP many times "NIB", though if it also includes the larger "PH included Box" the price goes up significantly. So its a bit of a toss-up.
Either way though, Zelda merchandise is almost always worth more than MSRP if you hold onto it. You might miss the peak of the value rise by not selling it at a certain point, but it'll always be worth something to someone. With enough collectors out there I'm sure you could hold onto it for 10 years and try selling it away from the glut of others who are selling theirs now and probably make more -- but with inflation and a "value" ceiling on these (as you mention also the outdated hardware), it could not have much further to rise...
But that's just my opinion -- I make no guarantees on market performance nor do I have a crystal ball!
