Direct quote 2014-06-12
“Originally I wanted to hand the series off to younger staff and let them carry on with it. And so we did, and that resulted in the game Metal Gear Rising: Revengeance. But after Rising, we found that for my younger staff, the numbered games were just too heavy for them. So that's why I decided to come back and work on them myself.”
What it documents
Asked in 2014 why he returned to Metal Gear after announcing he would leave it, Kojima explained that he had originally wanted to hand the series to younger staff and did so, which resulted in Metal Gear Rising: Revengeance; after Rising they found the numbered games were too heavy for his younger staff, so he decided to come back and work on them himself, and he added that ideally he would want them to handle the series while he focused on creating a different IP.
Source
TIME, by Matt Peckham; interview at E3 2014, 2014-06-12. View source ↗
“Ideally I'd want them to handle this, and for myself to be focused on creating a different IP.”