I'm cutting him a bit of a break: He is saying what he thinks WILL happen, not that Nintendo is not on top right now. Phones are becoming very powerful, and have Digital Distribution right from the get go, not to mention the consumer, "One device to do it all" mentality. The more people rely on phones, the less likely they are to make an investment in a hand held to play games that, even if they were higher quality, might not be higher enough quality to merit an entire investment. People replace their phones more often then handhelds as well, so Phones will likely outpace handhelds in terms of power. He also didn't so much say that Nintendo handholds will die, just that they won't be the leader. It's not crazy, but I think there is a few possible flaws in his reasoning.
Look at the Kindle, and to a lesser extent, other E Readers. They predicted that the iPhone and iPad would make it a flop. But then, they became a staple. How did this happen? For one, E Readers became super cheap, comparatively. Even if you need a new phone anyways, the cheapness removes that nagging doubt that for the same price, you could just get a new phone that can also function as a reader. Secondly, the Kindle offers something that fundamentally, the iPhone can't do: E Ink. That is a boon for a specialized reading product that the phones can't do without making it worse as a phone. And so, the Kindle carved out a niche. What does this show Nintendo?
Well, it shows that if they create a cheaper, specialized alternative, that has a capacity as a dedicated gaming device that it's competition just can't emulate on a more general use machine, you can get your demographic. Can Nintendo make a cheaper product that has a gimmick that pulls in the consumers? Well, history says that that is their specialty. I think Nintendo has a fight on their hands. But whether Phones cut into Nintendos share of the pie, or if people just start playing more games in general without cutting back on there Nintendo handheld consumption, is going to be an interesting fight to watch.