I've heard what Kojima has said, but I don't think he completely wants to let the series go. He's one of the most respected developers, he must be able to get funding for other projects if he wanted to. I think part of him is attached to the series and another part of him doesn't want to give the MGS series off to someone else unless he's sure they can live up to standard he has set. There is part of him that wants to do other stuff of course though.
Metal Gear Solid 4 : Guns of the Patriots! (4/15/06) Damn the Patriots!Sneak Out 6.12.08 - SNAAAAKE!
It's mostly the fans faults. Kojima tried to move away from MGS4 but he received so many death threats, and no matter what you are or where you are you should never take a single death threat as a joke. He stuck with MGS once again. I guess he might kinda enjoy it.
You can tell he's trying to move away though. The way Metal Gear Peace Walker played it could have been a whole new game.
Also Kojima reported that the next Metal Gear game will have many ideas from different new IP's he wants to work on. (If Phantom Pain is a sign of anything) that might have been one of the new IP's he wanted to do jut with Metal Gear skin coated over it.