Really interesting thread this, I have raced in the past XC, DH and Enduro (including the Mega) in all cases badly but have still enjoyed the experience. FWIW XC racing was for me the least enjoyable if I tried to do well, because it involves pushing your heart rate into the red and then trying to keep it there until the race ends. But then a great feeling once you have completed the course and managed to overtake a few people who went off too hard.
I have found both skill AND fitness to be a limiting factor in all formats, if you want to have fun fitness helps, if you want do well you have to have both. Obviously DH fitness and XC fitness are not the same once you get to a high level, DH requires much more explosive effort and more upper body strength, but the fitness base is the same.
It doesn’t matter if you are racing DH or XC if you can’t corner well and carry speed you will be slower and use more energy.
As for why we have so few top XC racers I’m not sure, we have many top DH’ers with no huge BC support so why no XC’ers I don’t know. Probably because no-one champions XC and its it is not pushed by the MTB media.
Most sports have lot of people who do it just for fun, most people who take part aren’t trying to conquer the world. It doesn’t mean you can’t have a successful elite level but it requires focus. DH focus comes from within I would suggest with people like Peaty and the industry nurturing talent, does this happen as much in XC in the UK? I don’t think so.