This is the way it has become in the gaming world. Comparing Gran Turismo with another long standing big title game, Gran Theft Auto, you can see how the development progress has slowed right down. GTAIII, Vice City and San Andreas all came out (on the PS2) within the space of 4 years. GTA IV and GTA V arrived (on the PS3) within 5 years. The current (PS4) gen hasn't even had it's own title, just a ported PS3 version. Ignoring Grand Theft Horseo (RDR2), we haven't seen a GTA since 2013. Seven years. The online versions of games make up for some of this gap as it keeps games fresher, but not every gamer is bothered by that aspect.
You can also look at Naughty Dog, another first party developer. Three Uncharted games + Last Of Us on PS3, just UC4+LOU2 on PS4. Half the output purely in number of games, on a console that is by all accounts far easier to work with than PS3.