Buying Quantic Dream makes actually more sense for Sony then buying any other 3rd party company avaliable.
Sony owns the IP to their Exclusive games anyway. If Sony decides to make a new Heavenly sword now, they don't neccesarily need Ninja Theory for example.
but Quantic dreams is different. they are the guys who made "Heavy Rain" but they don't go on making Heavy Rain 2 and 3 they make a different game, a new IP, but they will known for Heavy Rain.
And if they get hired by another company and make a new game, they are still the "guys who made Heavy Rain"
Their name is worth more then the name of the IP in my opinion.
“It appeared that there had even been demonstrations to thank Big Brother for raising the chocolate ration to twenty grams a week. And only yesterday, he reflected, it had been announced that the ration was to be reduced to twenty grams a week. Was it possible that they could swallow that, after only twenty-four hours? Yes, they swallowed it.”
- George Orwell, ‘1984’
