Steam already exists on the PS3, and steam is not a competitor as a platform. It is a service that allows you to buy and download games.
I believe that sony will go to download only in the middle of the PS4s lifespan, and having a company like steam that has already done this in one area would make the transition easier.
Now if they were to go to strictly download only games in the future, you would either need to be able to stream it or increase your hdd space drastically.. from what I have read, the ps3 is easy to upgrade when it comes to hdd space compared to the xbox..
But sony does have a company that offers the ability to stream any game over the internet to a console named in an above post.. Since they spent quite a pretty penny on them you better believe they will start utilizing them as soon as they can and as much as they can.
Just my thoughts