Well, look at it this way:
PS Plus was a service where the money paid into it by subscribers was used to pay publishers and independent developers for providing content to the service. The fact that they managed to get so much stuff on the service is all the proof we need that it was a good deal for Sony and publishers both. Now what do you think will happen when they put online behind the paywall?
They will get a lot more subscribers than before, and because of the epic failure that is the XBO they will not only be able to get the PS3 owners that weren't quite convinced of the value of the service before, but lots of former Xbox fans as well, and we all know that they are willing to put up with online fees. So, Sony will get more subscribers and consequently more money to give to publishers for content for PS Plus. Do you think this would lead to a situation where they would be able to put more content on the service or less content? Also, how do you think the fact that, contrary to the PS3, all games on the PS4 will be available digitally will affect their ability to offer digital content with PS Plus? Obviously, there is no back catalogue on the PS4 yet, so it will take some time for it to get going, but two years from now, I would be very surprised if the service is not at least as good as it is now on PS3.
It still sucks that we need to pay to play online though :/
The billion dollar used games market spreads awareness of game franchises to potential customers, puts more money into the hands of the people that buy games day one, and makes gamers more willing to take risks on unknown entities. Furthermore, I am willing to bet that ten-person game sharing would impact the sales of new games much more than the used games market.