50/50 is reasonable from Disney's point of view, because they'd be putting up half the money and doing half the work. But Spiderman is one of Sony's most valuable properties, so it would be like someone coming to Disney and saying "We're going to pay half the production cost of Star Wars, and in exchange you split the profits with us."
Thanks, but no thanks, we'd rather just pay for our own movie and keep the profits.
The deal was definitely in need of renegotiation because Disney was only getting 5% of first-day profits - but asking to become a full co-producer was, in my opinion, too big of a bite. And Sony is claiming that they tried to find a reasonable counter-position but Disney wouldn't budge.
At this point, what is Disney really bringing to the table that would make them deserving of that? What does Tom Holland's Spiderman gain from his association with the Marvelverse? They did a lot for the franchise at the beginning, especially since Sony needed to recover from having prematurely rebooted the series with The Amazing Spiderman. Marvel was able to legitemize it in a way that would have been impossible for Sony to do on their own.
But two movies later, it's now an established franchise. They don't need Disney's help to promote the movie, it's just a nice and cool thing for the fans. But since they don't need the financial help on the front end, there's no way they're going to cut their profits in half on the back end.
And in my opinion they're completely justified in feeling that way.
