Giving a game Platinum status usually means it has reached it's sales peak and now is leaning towards stability, in other words : The game is already old by industry standards.
It can also point out that a newer product is on the pipeline, it could be a DLC pack, another title or just nothing. Sony reduced the PS3 asking price too, maybe it's just some more agressive pricing policy and they're reducing the price of premium titles to go along with it.
The fact that matter is : Despite the decent sales figures, GT5 hasn't accomplished it's goal as a console seller. It will be quite hard to build hype on the community for the next game since Gran Turismo is not the benchmark anymore but hopefully, PD can excel again and give us a game as good as GT4 was when it came out. Here's hoping.
GT4 is very much more worst, it didn't work at all on my PS2 (slim model) GT4 is the single biggest mistake in gaming history i've seen. If it would work, i'd probably rated it as one of the better PS games, but i won't find out how good GT4 actually is i'm afraid, so -imo- it can't get more horrible than GT4.
But if they would upgrade GT4 till GT5 level (that is enhance the graphics & give cars interior view) i'd be happy but i guess i'm not a big enough audience for PD to get payback on such investment -LOL-
Beside my disappointment wit GT4 (sorry i had to mention it) i think your right tho.
And my best guess would be that GT5 wasn't ready when sony decided it couldn't be postponed any longer and PD was forced to finish GT5 in such a hurry that the quality was dodgy and the content rather small.
Imagine having no internet and buying GT5 ! That says enough.
Those things can (& must -imo-) be addressed in any future edition of GT.
I'd rather wait a bit longer and buy a game with the quality GTA or HalfLife have, perhaps PD should expand there workforce to achieve such. Or else we could be waiting for several years for the next PD game which has GT3 quality, or the next GT could become like dukenuke'm
Anyway you look at it you can't expect to keep everybody 100% happy, but you can't sell products like what GT5 was either, and i hope PD/sony learned from it and give GT the QA that it deserves.
Greetings stano666