A system can be owned using various exploits combined together. So if someone finds a way to alter the installation images, suddenly a mild trick that would normally just result in funny icons is now an exploit.
Maybe Nintendo never considered someone screwing around with their network to load an arbitrary image in a place that only Nintendo-Generated Mii png images should be but then I found a way... and if that libpng was vulnerable, suddenly we've gained usermode execution on a Wii U. Or worse yet, similar to the Sony PSP .tiff image exploit, maybe after gaining usermode we find a kernel-mode exploit and attack that. Now, we're near the realm of game piracy and private keys.
Kinda good example right here: https://www.youtube.com/watch?v=CkPAgv1Gjz0
Maybe Nintendo never considered someone screwing around with their network to load an arbitrary image in a place that only Nintendo-Generated Mii png images should be but then I found a way... and if that libpng was vulnerable, suddenly we've gained usermode execution on a Wii U. Or worse yet, similar to the Sony PSP .tiff image exploit, maybe after gaining usermode we find a kernel-mode exploit and attack that. Now, we're near the realm of game piracy and private keys.