That is part of the endless debate on whether historical players were better than the current batch. Personally, I feel that if you take a current player and time warp them back to the 1920's, they would dominate as much as Babe Ruth. The problem is that these players in the 20's, 30's, Aughts, teens, and so on were using the technology and training procedures in place for their day and time. Players today take advantage of much better work out routines, medicines, diets, and travel accommodations. Great players of the past would still be great players of today because they would have the same advantages as current players, but let's take Ty Cobb and put him in today's game going through a time warp. He isn't nearly as celebrated although he would be good due to his pure work ethic.
Having said that, I disagree with you saying Walter Johnson is the best pitcher of all time. He was the best and scariest of his era but would go to Satchel Paige as the best ever, even though he never got the chance to play in his prime on the big stage.