I think people are discounting the recent Qualcomm developments too much here. Their pairing of their new Snapdragon Elite chip with Windows, but also targeting high level desktop performance means it is likely to be a very viable contender for any future handheld, especially if paired with a Radeon mobile or Tegra.
But being an ARM SoC it wouldn't run your Steam library, which is what most people want in a handheld gaming pc. And if you use it to emulate x86 you lose efficiency, which is the main thing these other chips are competing for.
According to Qualcomm it would. It's extremely Early days in this, but the claims from MS and Qualcomm indicate ARM x86 emulation being quite good, faster in some cases. MS is throwing a lot of weight behind their "Prism" x86 to ARM instruction mapper/translator. And without adding much in the way of power draw.
I'm not gonna pretend you can believe marketing bullshit, but there is definitely at least some real development happening here. And if you paired the newest Qualcomm chips with a chip like the Tegra, you could have some amazingly battery life.
So it's no guarentee, but it's a huge development happening that at the very is going to shake things up even if it doesn't make a direct impact in the end.