This is my example:
The Don - LEGENDARY batsman.
Now, if he devoted himself to training specifically to face today's cricketers, in today's conditions...Would his Test average of 99.94 still hold up? I, personally, doubt it. And here's why:
Insanley-talented freaks like Muttiah Muralithuran, Shane Warne, Courtney Walsh, Flintoff, etc...They weren't around during The Don's era. Mind you, around his time, you still had some great cricketers - but how many could fair dinkum bowl a ball at 150+ kph, like Lee or Akhtar? How many could bowl vicious doosras or leg-breaks that swung and spun in amazing ways? In the words of Scribe, "Not many - if any."
I'm a big believer in the adage that a legend is a product of his own era. The great Magic Johnson would struggle to maintain his amazing match figures in the NBA of today, playing against guys like Iverson, Tim Duncan and the like. Maradona would have a much harder time dominating the game in this day and age against bigger, more physical and agile players.