He will get older or lazy and he will lose.
Father time is undefeated! If he continues to fight, his speed will decline and he will eventually lose. This has happened to every dominant fighter in history, with the exception of a few (Floyd,Marciano).
It was painful for me to watch RJJ at the end of his career, as I was a huge fan during his prime. I honestly thought he was truly unbeatable from 1995-2002. His hand speed was godlike, and his footwork was insane as well. He looked like a different fighter toward the end of his career. His speed was almost gone, which is what he relied on for most of his career.
If JJ stays in the game, this will eventually happen to him, and he will get his head knocked off. Although I will enjoy watching every second of him finally getting his ass handed to him!