Snake had to take a nap after a run in with Vamp in MGS2, so I refute your statement that two years isn't enough to degrade his acrobatics.
And while we're on the subject, how many combat-oriented cutscenes did you see Snake in Sons of Liberty, exactly? You were around him for about a third of the game, if that. Ray didn't exactly give him much of an opportunity to do much.
SPOILERZLOL This doesn't have anything to do with MGS4's progression of Snake's Werner's Syndrome.
If you'll recall, people tell Snake at regular intervals that he's not aging well and looks old. He's aging at a constant rate, and I'd wager that those two years are enough to keep Snake from wanting to jump off of any more missiles. Werner's syndrome didn't just decide to kick in a couple years after Sons of Liberty. He's been getting older the whole time.[/sPOILOLERZ]
Let's face it, over-the-top action is a part of the MGS franchise. Let's not forget that EVA does a Goddamn BACKFLIP with her motorcycle in MGS3.
If you don't want to read the spoiler tags, then just trust me on this one: an age difference of two years between the first two games isn't going to suddenly break his ability to pull those stunts. Your argument still doesn't work, and Twin Snakes is still just as flawed in this regard as it was yesterday. So you're still wrong.