That might not be so true anymore. A while back Rockstar and Sony said they would be working very closely together at least after the new GTA is released, on at least one PS3 exclusive. Looks like they were serious about it.
GTA 7 would be the one after the upcoming release if you count by number not by name as Rockstar only released a new numbered GTA when a new engine is designed. Who knows if it is completely true, but from previous releases from both companies it could be and would help Sony greatly especially with their recent price cut.