half of them would be the Belgians that follow WvA around. I don't know enough about specific 'crosser since broadcasts here are few. Their are plenty that stay close enough to the top guys to fill out the roster of a 2nd tier WC team if they were compensated to ride the road. Most crossers and MTB riders I've met are far tougher than road racers. Given proper training I have no doubt many would surprise the purists here.
I don't think calling folks who don't think the level of the XC riders right now isn't on par with the Road Peloton "purists" is fair. I'm a mountain biker, so could hardly be called a "purist" with regard to the road.
Clearly there have been XC riders who have come good on the road, legends even. Now and in past decades. However, for me the general feeling is that all the guys who can, have moved at this point. When MVdP, WVA, and TP aren't at a race, it seems to me that the level is just not close. I can't prove it. I'm surprised anyone debates it though, it seems pretty obvious to me. The idea that any of those guys could compete with a guy who does whatever he wants in the pro peloton, and is regarded as the GOAT or close...well I just can't see it. But we're all entitled to our POV of course.