If Disney buys straight up buys some obscure characters, Uni would get money for characters that they probably wouldn't use, anyway. Disney would get to synergize some of their upcoming Marvel projects for a lot less money than trying to buy the entire theme park rights back.
If Uni took a settlement to agree to a list of characters that Disney could use, if Disney came up with the right list of characters, it would simply be paying money to avoid a long, costly legal fight. Disney would also get some clarity in what is a fairly confusing situation right now (i.e. who is part of a "family"). Uni would benefit from this, too, as long as Disney didn't reach too far with their list. Uni's best-case scenario in a legal fight over some of the more obscure characters is that they pay their lawyers $$$ to retain rights to characters that they will likely never use in their land in any substantive way. Taking a settlement gives them extra money for avoiding a court case that they have x% chance of losing. Again, it would depend on what was on the list.
Edit: I think both scenarios are possible, depending on which characters Disney gets and for how much money. Just buying Uni's theme park rights altogether is straight up crazy, but I'm envisioning payments in these scenarios could be pretty cheap.