It will be interesting to see when Sony will pull the trigger on the PS5. They are in a great position with the PS4, a position they have not been in since the PS2 days. So, it is natural they they would want to extend the length they are in this position for as long as possible. On the other hand, waiting too long for the PS5 risks Sony giving MS as strong edge, at least I NA, where the XBO still sells competitively with the PS4. A few good moves from MS, and I can easily see a scenario whee the next Xbox have Xbox 360-like dominance/influence in NA.
Then you have to consider PS VR, which is a big wild card. If it becomes a hit, that's just more incentive for Sony to extend the PS4's lifespan.
As for first party output, I just don't think 5 years is not enough time for development studios to release no more that 1-2 big AAA titles. AAA game development is more complex and time consuming than ever.