I don't think he's wrong. I think the ultimate appeal of dedicated handhelds is going to be limited compared to mobile. I think the Switch is successful right now at least partly because it's squeezing every last bit of that market it can.
Right now, it looks like the Switch is going to ultimately consolidate the audiences of the 3DS, Vita, and Wii U. With all that combined you have a pretty healthy platform, but not a mainstream phenomenon on the level of mobile, the Wii, or the original DS.
As for the future of portable gaming in general, I think that lies in pretty much being an extension of consoles, which is what the Switch is a start for. In the past handhelds were successful because of great wholly exclusive games because of the massive differences in what they could achieve compared to consoles. That changed not only because of mobile but because with the advent of 3DS and Vita, handhelds became more or less fully capable of running the kinds of 3D games Japanese developers tend to make on consoles.
Here's what I see Sony possibly doing later on, if it's even physically possible: Wait a few more years until the original PS4 hardware becomes mature enough, shrink it down to about the size of a Switch, and just launch that as a portable system that plays all digital PS4 games. It would work even better if Sony gave PS5 developers the option to make games on the platform that are still playable on PS4, effectively extending the PS4's life. At the very least indie developers would definitely do this.
And if devices like the GPD Win catch on, real portable PC gaming could have some potential.