In 2018 there were 872 moves released into theaters. Disney, 21Century Fox and Fox Light released 36 of them. That is about 4 % of the total. One could argue if the other 96% produced movies that people wanted to see, (like they want to see Disney Movies) they too could have mega movie returns in dollars. There is no guarrantee that Disney and its new acquisition will be able to maintain its revenue dominance, and there is nothing preventing or limiting the market acces of the other96 % of the market. This is one consideration in considering anti-competitive ownership......
Also, Disney through Marvel already owns the rights to Spider-Man and other MCA characters which Sony controls. They have licensed Sony exclusive rights to make and distribute movies on these characters. They receive a royalty payment from Sony for every movie they make and distribute. So if Sony wanted to change the terms on an contract with Disney/Marvel on an asset which Disney/Marvel already own, there really wouldn’t be any purchase of assets.... Instead there would be a change to the rights Sony has to use Disney/Marvels current assets. (My theory is there are Not likely any Anti-trust issues, because Disney / Marvel already owns the characters)