They undoubtly have but you can pimp all you want, you cannot hide the fact the 'car' is from 2003 . . .
I don't think it's a matter of losing customers, I strongly believe its a combination of things and most likely the work ethic they have there.
I once read that Valve employees are allowed to do as they please there without set boundaries and deadlines. Just look at the SteamOS/Steambox and VR technology where they keep themselves occupied with, just because one or two blokes thought it was a good idea. Had Valve been subsidiary of (for instance) Activision or EA, then we would've seen a HL2 Episode 3 already, perhaps even more episodes or titles which would've destroyed the franchise.
On the other hand, letting your customers wait for almost 7 years now to continue the storyline is apart from being insulting towards the customer, also a franchise killer. Valve needs to come with a whole new engine indeed and will need to find a way in telling the story better then what they did with Half-Life 2, since the competition already surpassed them on several occasions, such as BioShock and Deus Ex: Human Revolution . . .