I think the main thing they've failed at in recent years is not game design, but PR/marketing and communication.
Other than not putting a muzzle on Kaz and hiding the Standard/Premium thing until the last minute (the latter of which is probably more arguable as good marketing than bad), there really wasn't anything wrong with GT5's marketing. But as far as the game itself, there was clearly an earnest attempt to improve things in the series, but they were almost laughably misguided. Even among the bizarre bouts of laziness from a AAA game that spend nearly half a decade in development, you can absolutely see the logic that went into things like the change of the game structure; just that no one ever stopped to think about any consequences for them. But Kaz's social media presence and interviews following the game, plus the game actually being improved fairly substantially (albeit strangely inconsistently) over the course of two years? That was truly brilliant as far as damage control goes. GT5 was always a bit of a mess, but they managed to shape it into a good enough of one and gave it enough support that lingering problems could be ignored.
GTPSP was the same. For a game that was supposed to (and in any other developers hands, would have) come out in ~2005 and had already become the butt of jokes by 2006, it whipped up a humongous furor when it was actually announced. And when you actually play it, you see there was an earnest attempt at making a game for a portable system; just created by someone who wasn't as clever as they thought themselves to be.
And GT6? How many people bought into it who ordinarily wouldn't have? "Look how reserved Kaz is in interviews". "Look how little is being promised for this game". "Wow, it is actually going to make its release date". "Look how frank Kaz is being about what needs to improve". There was a lot of positive sentiment for that game that wouldn't have been there if they had gone in guns blazing like they had wth GT5, and I know it got people to buy it who were otherwise expecting very little improvement and not have bothered. And how was anyone to know that the game would actually be fairly consistently a half assed, worse performing, even more bizarrely designed wreck than GT5; not the least after that game had two years of continual improvements?
It wasn't until after GT6 where the PR fell into a death spiral. Course creator delayed for a year and a half, cut down in features and external from the game with no explanation whatsoever. Partnering with a fan site to create a Q&A forum and blog, then pretty much abandoning it after they go over one question (again, with no explanation whatsoever). The huge paid post-release content announced for the game never even started. The VGT program getting delayed so long that it became irrelevant to GT6 so PD abandoned it in kind. Giving interviews about the development greatness of the next console and release date targets that end up making them look dumb when the game blows right by them; and are eerily similar to the ones they gave for the previous system as well. 2013 FIA series very quietly forgotten about. Completely shutting off any social media presence except for the guy who gives occasional half answers to questions and the blog that occasionally talks about how Kaz went to some car show or whatever. Having a public beta make up almost the entirety of your game announcement, then quietly cancelling it in the hopes everyone would forget with some vague statements of getting the game out in time, then delaying the game indefinitely anyway; none of which with any explanation.
Those are all some disastrous PR gaffes from a series that has been traditionally been pretty decent at it (GT5's delays aside) and you're technically right that they
have all happened in the last few years; but there's no reason to think that they belie anything about how anyone at PD has learned anything from the last three games they have released in terms of design.