Don't they own a piece of him that's there's forever unless they sell it away?
Its not that Spider-man is less important to Sony, its that *movie production* is less important to Sony. They've considered getting out of the business before. And if they leave the movie-making business, the rights head back to Marvel.
Whereas Fox isn't going to stop making movies, which means they won't stop making X-Men movies.
They would probably reduce the special effects and/or focus on the more ground level characters like Wolverine, and BeastI have to think that if they start making these movies at a loss they'll be forced to give them back. At some point people are not going to keep going to X-Men movies every two or three years.
They are paying for the rights to make Spider-man movies, but they do not straight up outright own anything re:Spider-man.