Because I continue to have too much time to think about things that interest me more than they should, I have some thoughts on the 2014 Tour, and what might have been in Froome had not left the race. I know this is well-hashed territory, but here goes.
In the 2012 Tour, Froome took almost four minutes on Nibali in 95K of time trials. The final (only) time trial in the 2014 Tour was 54K. Not a perfect comparison -- the 2012 TT's were flat, the 2014 TT had two hills/climbs -- and not sure if the hills/climbs would help or hurt Nibali, but in the absence of conclusive analysis, say it is a wash. Projected loss for Nibali for the 2014 TT would be around two minutes, by my reckoning.
So, the project for Nibali over the rest of the 2014 Tour would be to gain over two minutes on Froome. If he had that, he had a reasonable chance at winning, assuming he could stay with Froome in the mountains.
At the end of stage 5, the cobbled stage which Froome abandoned on, Nibali gained almost two minutes on Porte and Thomas, the highest-placed Sky finishers. Assume that with Froome there, the two could have worked together better, and reduced the gap to a minute. At this point, Nibali has one of the two minutes he needs.
Which means that Nibali still needs to take a minute on Froome in the mountains. At first glance, this seems a little unlikely, but I think a case can be made that Froome's performance over an entire Tour in the mountains is not as dominant as one might think. Admittedly, these would be unusual circumstances, as Froome has not entered the mountains in the Tour with a time deficit on a GT contender, but his history in Tours says that, apart from 2012, he has lost time on at least one mountain stage in each Tour he has ridden since then, and in at least two Tours, he has lost significant time on contenders.
This is balanced by Froome's outstanding performances in single mountain stages, particularly to Ax3 in 2013 and PSM in 2015, where he distanced the field by one to two minutes. But in both of these Tours, he also lost a fair amount of time in a mountain stage or two, to Quintana and Purito in 2013, and Quintana and Nibali in 2015. The point is not so much that Froome had a net time loss in the mountain stages, though he did lose time overall to Quintana in the mountains in 2015, by my calculations, but that he shows a lack of consistency in his performances in the mountain stages. Nibali in 2014 had one stage where he lost time to a contender, stage 8 in which he lost three seconds to Contador.
My point being that there is some history of Froome losing time on individual stages in the mountains in the Tour, and, in 2015, of losing time overall in the mountains, despite an individual performance such as PSM. It would have been a huge ask of Nibali, perhaps, but it was precedented, to some extent, and given the performance Nibali had in 2014, that would have been the year in which he could have put a minute into Froome in the mountains of the Tour. As a biased observer, I think Nibali could have put over a minute into Froome, but perhaps the larger point is that if Froome had not left the race, it could have been a spectacular battle between the two.