Its not about proving anything though. Does anyone not think DC is a top contender? He has wins over Barnett, Silva and Mir. He is arguably the #2 or #3 HW contender in the world.
With Nelson, you have a fighter with a name who can put on an exciting fight.
Who else could he fight? DC wants to fight this fall. Struve is coming off a loss. Same with Hunt. Miocic has win but does beating him "prove" anything. Overeem and Browne are fighting in late August so they wont be available. And DC has wins over the other 3 fighters in the top 10.
Werdum is the only other person who is coming off a win, has a name and who is ranked where Cormier is and I listed earlier why that cant happen. If Cormier was willing to fight Cain, this wouldnt be an issue.