While I agree 100% with the rest of your post, the downward trend is only one game (GT6) selling poorly compared to the rest of the series, and that's likely got a lot to do with the casual buyers, who would have made up the vast majority of sales of previous GT games, that probably weren't interested in GT6, considering it came out for PS3 after the PS4 had already been released. This will have had a big impact on sales.
Also, GT6 released with a lot of bugs, major online stability issues, a lack of new content over GT5, and a watered down career mode. The news of these kinds of things puts off potential buyers, you just have to look at how poorly Pcars has sold after the initial massive sales spike (due to the insane hype), thanks to the news of it's enormous number of bugs and issues spreading like wildfire.
Before GT6, PD had nothing to worry about regarding sales. GT5 was the second best selling installment in the series, outselling GT4 by a fairly small margain. If you count GT4 and GT4: Prologue sales together, the total is over 13 mill, but if you put GT5 and 5: Prologue sales together, it totals over 17 million!
So out of the over 75 million units the GT series has sold worldwide, around 22 million of those were sold on PS3, despite the relatively poor sales of GT6. For comparison, GT sales on PS1 totalled just over 20 million, and on PS2 totalled a whopping 29 million, thanks to two of the top three best selling GT games being on PS2.
If the sales of GTS are more in line with GT6, then it will surely ring alarm bells at PD, being the first GT on new hardware, but I'd be very surprised if it sells so poorly. Considering the massive install base of the PS4 already, and the hype and anticipation around the first GT games on new systems, I'd expect it to sell very well.