Sub 2 is only a question of when. People thought 4 couldn't be broken. Once the mental barrier is broken, it becomes possible. Consider the progression of the marathon. In a decade or two, the record has gone down 3 minutes in less than 2 decades. Almost 7 minutes in the last 4 decades. It will go down someday, the progression will be slow though. Maybe 2:02 by 2030 2:01 by 2050 2:00 by 2080? Who knows, but there's no evidence to suggest it can't happen other than that Bekele is the current GOAT and can't hit it. Viren was an king in his era and now he would be an also ran because of how much the sport has progressed.
Exactly who were these people who thought the 4 min mile was "impossible"? Gunder HÃ¤gg ran 4,01 in 1945, so I donÂ´t think anyone thought it was impossible after that.