Valve, like a lot of other companies, hopes that VR will become a thing, so they're making a headset. Because they'd like to sell headsets, they took a major property and made a VR version. If they lose money on their VR version of Half Life, they're fine with that.

Some game makers will surely try VR versions as well. This could be a big wave, and they want to be ready, so it's ok to lose money. But if unit sales aren't there, they're not going to keep doing it. In which case, things could well end up just like every other attempt at VR going back to Nintendo's 1995 effort. Or like the various waves of 3D movies and TV, where everybody's really excited for a hot minute and then people stop caring.