A gaming device isn't merely any device that can play games, but rather a dedicated device for the purpose of gaming. A TI-83 calculator can play games, but no one in their right mind would call it a gaming device.
That said, my point was more that the 3DS handily outsold the major gaming consoles, and consequently was an undeniable success.
And if anything, I think incarnedine_v's point about the popularity of mobile phone apps only proves the potential popularity of handheld gaming systems. Which makes me wonder - would it be brilliant or stupid if the next 3DS was also a phone / web browser?