I don't think that's true, at least when it comes to Pokemon Go.
Playing Pokemon Go has inspired me to dig out my ancient gameboy and to start playing Pokemon again. I'm strongly considering buying a new Nintendo device—I'd forgotten how fun Pokemon is.
Totally anecdotal but I had the same experience. Playing Pokemon Go prompted me to go on a nostalgia-fest and purchase red and green on VC for my 3DS. I wouldn't be surprised if the core game sales are seeing an uptick from the interest around Pokemon Go if for no reason other than it reminds those of us that were around for the original Pokemon surge.
Pokemon Go got me hyped up for Sun & Moon and the drive to finally go through Omega Ruby which I had sitting installed on my SD card forever but never bothered to beat the first gym.
Playing Pokemon Go has inspired me to dig out my ancient gameboy and to start playing Pokemon again. I'm strongly considering buying a new Nintendo device—I'd forgotten how fun Pokemon is.