Valve's Source engine is getting up in years, having debuted in 2004. Still, the company isn't keen on dropping the engine, and claims that updating and iterating on it makes for an easier workload than creating a "Source 2" engine.
"That's just the way we work with that engine, [we] just update it not just replace it," said Valve's Chet Faliszek.
While Faliszek told GamingLives (via PC Gamer) they may create a Source 2 "at some point," the current model is easier for the time being. "When you replace an engine you're replacing the tools and the way that people work, there's an expense in man hours and people learning and people getting up on it right."
One part of the iterative process was adding Mac support last year. According to Faliszek, testing on Mac hasn't added too much time to adding new features to the engine itself. "There's always some weirdness when we add new features to the engine, and we need to make sure when we do it still works with the Mac, but it's always part of the process."