Other than the H-bomb, Hendo & Sonnen are very similar. It would be an easier fight against Sonnen.
You must have forgotten how Jon "Spineless" Jones got his title shot. Let me refresh your memory, it was as a short notice(2-3 weeks)injury replacement. Injuries happen & fighters should be expected to stay on the card and fight the injured fighter's replacement.
Whether you want to admit or not, Sonnen as a replacement sells. Heck, Sonnen might sell better than Hendo.
As for the co-main event, this wasn't the original fight. After injuries, we had what we had. There wasn't much the UFC can do about injuries except find replacements.
the opportunity cost of taking a fight is significantly increased once you become a champ. that's why so many start to “fight safe” when they have the belt. sonnen wouldn't fight anyone like now he does if he were a champion.