Even as a peripheral I'm not sure there's a big market for this, Wii U demonstrated that off TV play isn't something that the mass market find terribly exciting.
Yeah there really isn't. If they're developing this with the expectations it will only sell to some enthusiasts and reach 2m lifetime on a good day then that's fine. If they're expecting this to be a mainstream device they've made a mistake. Really if this is a misstep or not depends entirely on what Sony are expecting from this.