I think the average number of games will remain the same, but they will be split over another platform, so those who are just looking for games on one platform, will in fact see less games for that platform.
It is very, very clever if you think about it, because being a subscriber encourages you to own all the platforms the games are coming for. How many people have looked at getting a Vita because they are already a PS3 PS+ member?
I think this will drive PS4 sales in small part. As of release, PS3/Vita owners that are also subbers will be 'paying' for PS4 games as well. They will want to play them, and it gives a push to own the extra systems.