@daus,
I'm deriving my assessment of Polyphony based on my understanding of the Japanese mindset, and Orientals in general. These are the people who have kids who will commit suicide if they don't have a 4.0 average. In middle school!
While the Oriental culture is all about humility, courtesy and politeness, they're also fiercely competitive, at least concerning ideals. Everyone wants to be perfect, behave perfectly and make perfect stuff. Kaz anguishes over every GT game they produce because he's not just in competition with the other developers, but against his own perfect dream racer. They're used to the acclaim lauded them over every GT release - and true no Gran Turismo will ever score 100. But with many, GT games are held to be one of the pinnacle racers in all genres. GT5 wasn't quite as well received by critics, and this has been the first GT game Kaz has apologized for a number of times.
And in the case of Forza, I know some get rather irritated when I say this, but Turn 10 and Microsoft basically took the entire concept of Gran Turismo rather than spend the time to come up with something unique. As with almost everything Microsoft does, they copy what's successful, and no racer is as successful as Gran Turismo. And this is something Kaz and the team have no doubt taken personally. Between these two factors, it's my opinion that they will fuel the ambitions to make GT6 much greater and more perfect.
As for Snaeper and Toronado,
I fixed that for you. (Because for a second there you sounded like you know what's going on at the head of every video game studio and production company at this date and time and that's pretty unlikely).
And...
No, we don't know. So why everyone is expecting more, rather than less (The DLC has disappointed everyone with it's car selection for example)
Ahem. Speaking as if you know what everyone thinks?
Apparently it's doing so well - the Touring Car DLC mind you, that Kaz made the effort to get SONY to permit him to make it available in the Dealerships for multiple purchases in December. If no one wanted it, I doubt he'd waste his time on it.
While some of your points are plausible, others are not. On graphics, you and I both would have bought GT4 HD, and so would hundreds of thousands of people, perhaps millions hungry for a next gen GT game. It's hard to say what the gaming public's reaction would be to something like that.
But lookit, even with Prologue looking just about photo-real, critics and many fans made a point to remark on how bad the shadows looked, and that they needed to be fixed. The shadows in GT5 look worse. The particle effects mess up everything, not just cars as I mentioned above. There is at least as much slowdown and more screen tearing, and this has been a point of contention with many, even fans who adore the game. So can you imagine the flaming that would result if GT5 looked like a PS2 game in HD? PC sims for years now have been criticized for similar graphics looking terrible. From the way some critics put it, you'd think these racers looked like early Genesis games.
If you think graphics don't matter, that's certainly an opinion that isn't widely shared. If rFactor 2 looked the same as rF1, you can bet the graphics would be scored as Poor to Bad. You're not going to find that the PS4, XBox370 or whatever with graphics that are anything but a big step up in quality. ATi isn't going to scale back on graphic card development and either leave the state of their GPUs the same or trimmed back. Calling them "green cards" sure won't help.

No video game without high quality graphics on anything but the Wii will be considered worthwhile by critics or gamers. Or as fun. They add to the immersion, they enhance satisfaction, and you can say otherwise all you want, but almost no one will agree with you. Sure, I love my Genesis and SNES and will play them from time to time, and in some cases, the gameplay is still superior. But those graphics are still cheezy, and I enjoy them simply for nostalgia's sake. I sure wouldn't pay $50 for a new game with those graphics.
As for the issue of DLC, this is a moot point. Most games have a great demand for DLC, and it feeds the gamers for a year to three after a game is released. And in many cases, in just about all cases in series, there will still be sequels. The DLC doesn't hurt the sequel's chances, sometimes it causes problems requiring patches - look at some of GT5's updates. The only effect that GT5 DLC will have on GT6 is buying Polyphony time to work on the sequel, even though most or all of the DLC will be in GT6. Every sequel stands or falls on its own merits, and has nothing to do with how much DLC adds to the previous game.
As for what DLC will mean for GT5, who knows? But what did it mean for Prologue? Have you guys forgotten that the entire online structure came about because of an update? A Livery Editor? It can happen, even something simple like a series of templates is better than nothing, and what we have now is basically one template which we can paint one region of. Better damage, slightly better graphics? Anything is possible. I don't know what Kaz has in mind for GT5, but I wouldn't rule anything out.
As for how interested the gaming public still is in GT5, amar212 has mentioned that it's apparently still quite strong. He said that one year after GT5's release, there are consistently tens of thousands of online racers, which he said is rare for online games. I expect GT5 to sell tons this holiday season, and next.
As for whatever platform GT6 ends up on, that's Kaz's decision. We'll buy it and race it, and if it's on PS4, we'll buy that too. Most of us.

And most likely it'll be a Platinum level pre-seller as Prologue and GT5 were. And while a GT6 Prologue isn't really necessary with GT5 around, we still might see one. And it too will sell like tacos.