Mayweather is old and McGregor is young. They are competing in Mayweather's area of expertise. McGregor has some experience in the sport, but nothing close to Mayweather's level. He's very good at a related, but not identical sport.
So Bolt vs. Farah doesn't fit.
van Neikerk against a 35 year old (two years past retirement) Edwin Moses at the 400 hurdles is the sort of thing we are looking for, I think, but we can't do that without a time machine.