If they feel they have the leverage, they don't need to offer anything (but the reality is they proabaly are willing to offer something).
We've known all along that if there is a new deal, its not going to be a situation where you can look at the old deal and do gives and takes.
Pixar's leverage in the deal has increased significantly since the first deal, and Disney's has decreased.
Therefore, this deal is going to swing much more favorably in Pixar's favor.
What's likely is that if Disney gives up ownership, they will ask for a 20/80 split instead of 10/90, or whatever is on the table at the time. Or maybe they will ask for better terms in acquiring merhandising rights back from Pixar. We don't really know, and maybe never will.
Really, just 2 points here:
1- There's nothing wrong with putting ownership of the already created films on the table for a new agreement.
2- I'm sure its a gross over-simplification to say that Pixar has drawn a line in the sand on the ownership issue and is not willing to discuss any other issues. But even if they did, that's still negotiating. They are offering a portion of the profits from their future creations.
Whether that's the best tactic is debateable, but its not like Disney isn't skilled in the art of "tough" negotiating.