"Instant Mix" said:12 yearscycle might be at its end! On a more serious note, I do think that Half-Life or some story that is intertwined with the HL1/2 universe will make an appearance in the years to come. These things always happen. A lot of stuff is recycled in our world, or inspired from old stuff. First big example would be the renaissance. We can apply the same pattern to everything cultural. The HL games had such a big impact that there just needs to be more time left for it to be forgotten. I also think that economical reasons are at play. People who run Valve are also humans and probably the period 2007-2008 when the house-market crisis happened wasn't a particularly good time to play it risky with a game that could both bring little revenue and critical fury. I don't know if Valve has shares that can be bought on the stock-market or the shareholders situation but in such cases we know the drill: shareholders want max profit.
[EDIT] I'd really like to try Titanfall 2 myself someday, some holiday. I just checked it after Dimbeak mentioned and - holy cow - that is a Source engine game! - although, somehow bizarrely you can notice that specific "plastic" feeling of metal surfaces (I watched a gameplay) Source engine has, though the natural environment seemed lush and really not Source engine-y