Back in the days ken was amazing, and no one could beat him, he was doing the least mistakes.
Now, Mango is the best, doing the least mistakes. He's not perfect or anything, just better than everyone!
If he ever quits, someone else will take the lead and we may have a discussion about who would be the best between player X and Mango, if he ever was playing again. But still, Ken doesn't play anymore, he's not among the best players in the USA. Would happen the same for Mango if he quits today. In a couple years, he would not be among the best.
If they ever play again, it's a bit ridiculous to consider if they would catch up or not... They showed great expertise in adaptation, maybe they would catch up at some point, depending on their will. Being the best again, after a couple years, we will never know.