I was actually pretty impressed by CastAR. It's certainly the most compelling AR-esque experience I've tried.
It opens up a different set of game possibilities than a VR headset like the Rift. Think augmented multiplayer tabletop games rather than immersive worlds.
I can see a single mind-blowing tabletop demo selling pretty damn well. The audience isn't huge and mass market, but it's decent sized, and willing to drop very high amounts of money on cool stuff.
I'm not sure the market is that slim (a lot of people in my social circles are playing them these days, certainly more than 5 years ago), and these board and card games are sold at rather expensive prices (€80 for a boardgame is not rare).
That said though, when I play a boardgame, I'm just playing it on a table with some friends. I don't want the games to get any more complicated, and I certainly don't want to put on headgear or glasses and involve technology for some gimmick AR feature. You'd have to make the game concepts that require such a setup very compelling for me to consider them.
If they had been doing Oculus Rift type stuff I'm sure they'd still be at Valve.