How does one tell when they have reached adulthood? They don't care about sales numbers, they just enjoy the system they bought.
I agree 100% but I do have an interest in the video game industry so I do like to keep tabs and play armchair CEO. Personally, I'm not saying "ha, it's failing!!" Matter of fact, a long-time favorite console by many people was the one whose eventual downfall took down an entire side of Sega (though in all fairness, the responsibility lies more with the Sega CD/32x/Saturn/Nomad/Pico/etc.).