Nintendo president Satoru Iwata can tell you when Nintendo would stop making its own handheld systems. He can tell you, hypothetically, when a line of machines that has gone from Game Boy to DS to 3DS would end and Nintendo's games would appear on the hardware made by other companies instead.
Last week in San Francisco, he told me when that could happen. (Hint: do not hold your breath.)
"I think that if we are able to provide experiences on handheld devices that consumers cannot get on another device, then we will continue creating software and hardware going forward, and if it comes to a point when we're not able to do that, I think, yeah, you will see portable handheld gaming devices go the way of the Dodo, I guess."
I've played games on both my iPhone and on the DS/3DS/PSP, and while iOS games are fun they are not games that I spend a extended amount of time playing in one sitting. I have found I prefer the feeling of actual controls- it was great seeing Wolfenstein 3D and the like on the small screen but the touch screen controls never matched up to the tactile feel of a d-pad or thumbstick and actual buttons.