People continue to overestimate the importance of MGS4, and underestimate the importance of GT5.
Look at software sales for MGS3/GT4. Look at hardware sales for the times those games were released. MGS3 gave no noticable bump in hardware sales, and sold 4 million copies; compare that to GT4's consisten 10m+, and the Metal Gear franchise isn't nearly as close in terms of significance.
Metal Gear is a franchise that's really, really big with otaku/internet crowd, and this gives the impression that everyone else cares about it just as much, but they don't. Compare this to something like Halo or Grand Theft Auto or Gran Turismo, which are games that actually do have significant effect outside the super enthusiast gamer crowd.
Please note that I'm not saying the Playstation 3 doesn't have good games next year; it absolutely does, I think it has the best lineup of system sellers next year. I'm just pointing out that a lot of people don't know which games are the biggest and which are still big, but not quite as big. Pin your hopes on Gran Turismo, not Metal Gear, in my opinion.