Cobalt
I don't see your logic here (entirely). When you buy 1 game, it has the same gameplay and relative content. So if 3 episodes contain the same content and relative gameplay as a full length title what's the difference. This argument would only hold true if they changed nothing at all in the course of 9 episodes. Which they have already stated is not going to happen. They plan to make adjustments, add new features and content with each episode. Of course this is all words. Until it happens we won't know.

You are making an assumption that the technology will not change. I think that's a major oversight. The Source engine is going under major revisions atm. And will over the next few years. Valve is moving towards episodic content, heck they've essentially been testing that model for 5 years now with HL1. Constantly adding bits here and there (for free). Using HDR as an example, this was a major rendering addition. More is coming, be sure of that. And unless Ritual does 0 changes to the renderer themselves, which I doubt, expect that they plan to do more with it. Also they are working on their own tech.