This is it. 100% correct.
Sony and PD are in it for the money. It doesn't matter if they keep pushing out updates every month or if they never touch the game again; the game is on shelves, the majority of the money has been made. They have no interest in what happens with the game at this point. The ONLY reason you might continue to see updates to GT5 is to keep the fanboys on the wagon so they will shell out another $60, $70, or more when GT6 hits shelves. But let's be honest, if you're still playing GT5 at this point, you're going to buy GT6 regardless.
The simple fact is, spending additional time and resources on GT5 at this point is poor business practice. PD essentially has 3 choices: Take resources away from the development of GT6 to improve GT5 (extremely unlikely to increase profits); drop further development of GT5 to begin work on GT6 (IMO, best choice if they don't want GT6 to be another GT5... 5 year development and an unfinished product at release); or abandon the GT project entirely (highly unlikely - Kaz is in this for the bank long haul).
Sony and PD are, after all, businesses, so it would be naive to expect them to do anything that isn't in the best interest of profits.