I would bet my life on the game being inevitable. VR is only more fuel on that fire since Half-Life 2 has been central to their tests with Vive.
I don't think they have the ambition to do it anymore. Everything I've read in recent years points towards money-hungry Valve being real. Projects that focus exclusively on making money with minimal risk. Over a third of the company is working on VR alone. There's no team hidden away working on HL3, and a lot of the key developers, artists, and writers from the HL teams have left, probably due to their being no timeline for another game.
Valve is all about multiplayer games with transactions now. It's kind of disgusting.
Do the math on how many copies HL3 would need to sell to outstrip a month's worth of Steam revenue. Now consider the risk to Valve and the amount of time and resources.
They may end up releasing a VR project similar to the Portal one, but HL3 is far from inevitable. They've also said that they'd never make HL3 VR only.
Yeah, if a developer is investing in VR it has to be a cash grab. No way could it be that a new technology has inspired genuine excitement in a large portion of the company. Nope, they're chasing dollars.
But the Steam revenue won't dry up if HL3 comes out. It's not an either/or proposition.
What exactly is the work that goes into dota 2?
What do they do, make some new skins, maps, add a couple new characters every year? I dont play these games but it can't be that time consuming.
You're confusing literal VR tech with VR games and experiences.
Valve isn't a large company, despite being wildly profitable. HL games take a very, very long time to make, even with max effort. I encourage you to read this article on how arduous the making of HL2 was. Now imagine a game that requires even more innovation, technology, effort, and risk. The incentive is simply not there. If it was, Valve wouldn't be hemorrhaging creative talent long associated with the HL franchise.