I just watched UFC fighter Francis Ngganou lose a controversial split decision to Tyson Fury, the undefeated heavyweight champion of the world, in his first ever boxing match. There have been threads here with people claiming Fury is the greatest ever and would have been too big for the likes of Ali and Mike Tyson to handle.
This is the equivalent of Ronaldo or Messi in their prime challenging Rudisha over 800m 10 years ago and losing on a photo finish after maybe 6 months to 1 year of specific 800m training.
It's possible there are just better athletes in MMA than boxing currently. But we've always known there are better athletes in soccer than athletics. Ronaldo at nearly 40 earns a million dollars a week, which is probably more than Marco Aprop has made in his career.
I could definitely see a soccer athlete on the level of Ronaldo in his prime beating somebody like Arop in an 800m challenge race, with 12 months training, especially if Arop or whoever took it a bit lightly and came in at 1:45 shape, like Tyson Fury probably just did. But they would have to offer the soccer player a billion dollars or such.