I suppose its not entirely unfathomable. Apparently he still gets on the mat and helps with the kids. And I think a 50 year old comeback would go better in Greco than it would in freestyle so he'd have that going for him. But he is starting to look and move like a 50 year old man. Could he beat someone of the world stage? Probably. But to beat a wrestler at the world championships he would be beating a national champion which almost always implies extremely skilled, extremely athletic, extremely strong, or all of the above. Wrestlers attempting comebacks at or near 40+ have had varying levels of success (Cary Kolat, Kendall Cross, Andre Metzger, Kurt Angle, Rulon Gardner, etc) but none of them have made world teams. I did a quick youtube search to see if I could find what Karelin moves like in 2018 and this is what I found: