There’s nothing Disney’s new company can do to outright prevent other studios from having successful movies, but it will certainly put Disney in a position where they have a level of control of the industry that no studio has ever had before.
Even once the deal is done, which hasn’t happened yet, it will take some time before these changes really take place, but you can be sure they will happen.
Certainly, market share isn’t the be all, end all.
While there are certainly elements of the Disney/Fox deal that are potentially great from the standpoint of a movie fan, like seeing the X-Men join the Marvel Cinematic Universe, from a business standpoint, the deal is a bit more concerning.