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Igor Anton was in the red jersey ahead of Nibali, but crashed out of the race.
Hard to know how either Wiggins or Hesjedal would have challenged in 2013 had they both not got ill, but they had both won a GT the previous year, and were both among the favourites. The GC just ended up a complete no contest, like the 2014 Tour.
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Igor Anton wasn't going to win that Vuelta, even if he'd remained in the race. I don't think he's ever finished in the top ten of a GT.
If Nibali had won one or two GTs, your argument might hold water, but to win four, and be one of the 6 riders who've won all three, suggests he's won them because he's a great rider rather than a lucky rider.
Who dropped out when he won the Vuelta? When he won the Giro in 2013 do you seriously think Wiggins would've beat him? He (Wiggins) was struggling to cope with the monastic existence required of a GT winner and his form was nowhere near his 2012 form.