The closest anyone can get to being unbeatable is if you are either Pac, or Floyd cos Floyd dont fight, or Bradley if you get gift decisions like he did against Pac.
But Khan can get harder to beat for sure. But the Garcia punch that landed on Khan in the 10th round rather than the third. So even if khan avoids that punch for another 6 rounds khan is still questionable.
What is interesting is how khan handles being hurt in future? Does he hold or try punch his way out of trouble.?