I fail to understand why the console has to keep the disc spinning AFTER a game has been fully installed and has been launched (initial disc check makes sense). The fact that we can play all the major releases from PSN too shows that there is no real technical reason. A "full installation" is possible. And performed anyway.
So why does the console keep the disc spinning? It would safe power, it would be much more quiet and it would go easy on the hardware. Moderators, please relay this question to the technical department 
In my opinion, it would be the most desirable improvement to my PS4.