Just to be clear on how the deal worked, the 5% split was not part of the MCU deal. In fact when Marvel sold the rights to Spider Man to Sony in 1999, the contract stipulated 5% of proceeds to Marvel and split merchandising between Sony and Marvel. This meant that Marvel was getting 5% without doing anything.
In 2011 Sony sold its merchandising rights to Marvel for $175m and $35m for any Spider Man film after as well as Disney giving up their 5% of movie proceeds. This meant Marvel was getting all the profit from merchandise for $175 + $35m per film for doing nothing related to the films.
In 2015 Marvel and Sony came to an agreement to allow Marvel creative control over making Spider Man movies with Sony financing, the only financial change to the past deals is that if the movie grossed more than $750m that Marvel would owe less than the $35m typically owed for a movie due to the merchandising deal. This meant that Marvel was now producing and running the show creatively, but were not gaining anything more than if they had not made the deal except for potentially a $35m discount (not sure on full details of the discount). With this arrangement, Marvel was basically giving Sony free money from their MCU involvement.
Looking at the above, Marvel/Disney didn't work on the last two Spider Man movies out of the goodness of their heart. If the movies do well, the merchandise will do well and make Disney money, but merchandise can only make so much and is indirect. Marvel could have sat back and continued to make profits off of the merchandise without spending their production time on making movies that Sony claimed all profits from.
Can you tell me that if you came in and helped prop up something to do better than it had ever done before that you'd want to renegotiate for a better deal? Is 50/50 fair? I really don't know, but Sony didn't counter offer and unless something changes Spider Man no longer has the MCU tie ins and creative leads to help it do better.
