As has been said Valve is a weird anomaly in the world of video game developers. They have the structure (or lack of rather) and resources to do whatever the hell they want. Also I wouldn't call a company that is trying things like supporting linux gaming, trying to bring pc gaming into the living room and working all kinds of weird biometric stuff as a company afraid of progression, more like a company that views progressions as more than just a straight line of improve game engine and make new games and sequels.
Granted the Source engine is starting to show its age a bit, though I actually think Source engine games have held up quite nicely over the years (well except for that 2012 thing...). I would be surprised if we see the successor to the source engine soon. Though I do find the thought that there is just one guy sitting alone at Valve who really wants to make a new engine and has done all the work himself quite amusing
Granted the Source engine is starting to show its age a bit, though I actually think Source engine games have held up quite nicely over the years (well except for that 2012 thing...). I would be surprised if we see the successor to the source engine soon. Though I do find the thought that there is just one guy sitting alone at Valve who really wants to make a new engine and has done all the work himself quite amusing