Didn't you need a lot of Fultons to S Rank all the missions? If not, I sure wasted a lot of time!
Peace Walker streamlined a lot of things, to make bite sized MGS experiences on the go. I thought it was great for those 5 or 10 minute breaks. The game was pretty easy already, so if you had the crawling perspective, it probably would ruin the challenge completely. You also had to wrestle a bit with PSP controls, so that's another reason why the AI was probably dumbed down. With the more precise controls and increased number of buttons on the PS3 port, it took a lot away from the game. I never played Twin Snakes, but wasn't there a similar problem giving the First Person Perspective and smoother controls?
In any case, I don't think it could have ever been MGS5 like Kojima claimed, with all the limitations of the system. He takes a lot of pride in work, and I think it would drive him nuts, if he couldn't use the best resources possible. I know he was really bummed with the sacrifices he made with MGS4, overestimating the ps3's power.