I was discussing this with a friend the other day. I don't think Sony's plans will change much, if at all, when it comes to expansion. Regardless of what happens with Xbox, Sony likely has their eye on a few things.
For one, they want more IP to fuel their transmedia ambitions. After the success they've had so far, they're probably even more emboldened to pursue that.
Another is future proofing their gaming business. They may be incredibly strong in the console space right now, but that may not be enough in the future to maintain much strength in the industry overall.
Sony likely wants to have a very strong first party so that they're not only NOT at the mercy of third parties and whatever may happen with them, but a possible future where console hardware just doesn't matter and they need to compete on other platforms they don't control.
Better to work on that now than after the fact. What that looks like, I have no idea. But certainly this little surprising success story of a multiplayer game, coupled with the previous entry being solid, if overlooked, has them looking at Arrowhead as a possible acquisition before someone else takes a shot.