It wasn't impossible for Contador, but it was in 2015. He could've done it if he had tried it in 2009, or he could've pulled it off in 2011 with better luck.
Contador was pretty much unequaled uphill in his best years, whereas Froome had his equal uphill in the 3rd week of a GT, and Nibali, Quintana and Contador were all really strong in most of these years.
Valverde's shenanigans in 2016 show the requirements for the double more than a single dominant GT. You need to be the kind of rider that can be good easily for long periods of time. Especially for Giro/Tour, where you enter the double with very little racing compared to Tour/Vuelta, which is a thing that's pretty underrated imo.
If Froome could have ever done it is something we'll probably never know. Tour/Vuelta doubles are too different to compare.
Contador was pretty much unequaled uphill in his best years, whereas Froome had his equal uphill in the 3rd week of a GT, and Nibali, Quintana and Contador were all really strong in most of these years.
Valverde's shenanigans in 2016 show the requirements for the double more than a single dominant GT. You need to be the kind of rider that can be good easily for long periods of time. Especially for Giro/Tour, where you enter the double with very little racing compared to Tour/Vuelta, which is a thing that's pretty underrated imo.
If Froome could have ever done it is something we'll probably never know. Tour/Vuelta doubles are too different to compare.