Repeating what others have said, but picking up Arrowhead seems like a wise decision for Sony to make based on the performance of HD2. They have no IP to their name, so it works out for Sony, but being an in-house studio also open the studio to a lot of additional help etc.
I can't say for sure how much the MTX is actually doing for Arrowhead and Sony, but if it's making a decent ROI then it's something that can be evaluated for other teams working on live service titles as well.
And while I fully prefer extra content to be in-universe stuff, being a Sony studio will also mean that crossovers with Sony IP will have a more permanent place in the game.