The government doesn't care about "horizontal takeovers". Anti-trust legislation is meant to stop "vertical takeovers", which creates monopolies that damage public interest.
There are dozens of film studios of different sizes, producing at least a hundred films a year (usually more). Disney is currently the largest and most profitable. They could buy Sony Pictures and WB Studios both and it wouldn't be considered a true Monopoly.
To use an analogy; if you were KFC, and you bought Popeyes and Chik-fil-A and Church's, you aren't a monopoly.
IF you buy up most all of the chicken farms, the feed producers and the restaurant supply companies, THEN you become a monopoly. The magnates of the early 20th century were doing this shit, buying up the steel plants, the coal mines that fueled them, the rail lines that moved the ore and fuel, ect, til you couldn't even try to make steel to compete with them.
This is why Time/AT&T gets scrutinized, they are a vertical takeover, owning the studios, AND the infrastructure (cable lines/satellites/transmission towers). If they started purchasing the big theater chains they'd be a certain monopoly.
All of Sony's old franchises added together and sent to Disney means nothing.