Do you think athletes of the past would be less impressive today due to improvements in facilities and knowledge of nutrition and exercise?
No, they'd be MORE impressive.
They were the best then, because they trained the hardest and competed the best. They had great genetics and used the latest training/nutritional technologies of the time.
Today, with all the advancements in training/nutrition and pharmaceuticals, these past great champions would still be great, and probably much better than they were in the past.