Look I agree that online multiplayer on the PS3 was a great selling point for the PS3 and the PSN. But that was because the PSN was behind the XBL service and still is in a few ways like cross game chat etc. Next gen will be a different animal all together. The PSN has grown in leaps and bounds and the PS4 will have those other features not to mention consoles are seemingly finally going to put more resources into dedicated servers etc. and that is going to require additional revenue for Sony to continue to expand and pay for the additional cost of those services.
PS3 remains free as it stands and most likely will stay that way.