It's weird that you say that because there are only a few other franchises in gaming that have maintained similar or growing sales with each iteration. Halo, FF, MGS, GT, GTA, and RE come to mind.
I also think it's not a question of making new IP, it's more a question of quality. Can we really say that IP like Elite Beat Agents, Electroplankton, Brain Age, etc. are in the same calibur as Mario, Zelda, and Metroid?
Pixel Art can be fun.







