FK I give you major props and have a great deal of respect for much of your shared opinions, and as much as one can via a message board, I consider you a friend... but I have to call you out on this issue.
I knew this was coming.

I'm a DLC whore when it comes to things I love. Hopefully, I can explain to you why GT is different.
After all, you have shown very little restraint in regards to
buying dozens upon dozens of DLC songs for Rock Band and SingStar, of which are often as much as $2 a song.... and frankly I would gladly trade in a 3-min song that I might play many times for a favorite car that I might enjoy several hours worth of gameplay with.
I will save a lot of quoting and just go from here.
Never once did I say I would not buy GT5 if it were mainly DLC based, I just wouldn't like it as much. And I wouldn't rush out to buy it at launch...I don't think.
The difference between Rock Band and and GT is in the nature of the game. Never mind that the disc-based content of Rock Band already meets the standard set for music games of this nature, and all I ask is that GT5 meet the standard they have been setting for themselves in content. That would be an easy out.
What I am arguing against is paying $60 for something with the content of Prologue and then being asked from day one to pay extra if I want my hundreds of cars. You can give me GT3 level of content on the disc and I will be happy.
Now, even then I would still react differently to GT DLC than I do Rock Band DLC. Rock Band emulates music I know and love, or don't know and/or hate. Even when I don't know the songs on the DLC that week a quick You Tube search will present me with the song and the ability to determine if I would enjoy playing it on toy guitars.
But GT differs from this in something I have already pointed out in this thread. Having all of the cars available up front allows me to see something I don't recognize, or overlooked, and then check it out. If I don't like the car I have suffered no loss. Now, if the cars are mostly available in DLC one of two things will be a likely outcome. 1) They don't even bother with little <100hp sub compact oddities and the like, which tend to be the cars that grab my attention like this. 2) The little oddities will be an unknown to me. I can't go on You Tube and check out if I like it like I can a song.
And am I supposed to take the suggestions of others on here to make my purchasing decisions? Who else can tell me they have had station wagon races, mini van races, etc with friends? Would anyone have told me the Nissan Pao is a great buy, even if just for spits and giggles? Better yet, would I pay $0.99 or $1.99 to get a Nissan Pao, knowing I will mainly just be doing hot laps on the Nurburgring?
If all I wanted was Super/Race Car Challenge then I would see this as a great idea, but I can get that in any game. Nowhere else can I get tiny unheard of (in games) gems like a Nissan Pao or original Beetle.
I can point at every version of GT and point out some car I gained respect for that I either had no respect for, or had never even known it existed. If they were done in a microtransaction form I wouldn't have ever found those cars. In fact, if GT1 had been done in a microtransaction form I would have probably not gotten a PS1, and thus my gaming would still be on a PC and I would be on some Starcraft forum discussing Starcraft 2, and doing all my racing in GTR or LFS.
No, the great thing about GT is the discovery. If I am limited to only discovering when I pay as I go then what is my incentive to discover? Suddenly it is a gamble. If I like my purchase, yay! If I don't I am out cash that I wish I hadn't spent.
I am not being some miser about my nickles and dimes, although I did bring it across that way. In my defense GT has given itself a content standard and my initial reaction to changing that would, and for this discussion, has been, negative, but I would get over it. But it would not be the same game.
Kazunori Yamauchi has said that he wants Gran Turismo to be an encyclopedia of cars. If it is microtransaction based it would lose that.
The experience from Guitar Hero into Rock Band did not change when DLC was added (even though I started with Rock Band). The content on the disc was the same as before. The DLC expanded the experience and allows the user to pick and choose how they wish to expand it. It also made the ability to increase song selection happen much quicker than waiting for the next version to come out did.
If GT5 had a few hundred cars and two or three dozen tracks and then had DLC I would be fine with that. I am against this notion of a Prologue-sized game that is then expanded by DLC, because at that point they are making more off of me than if they had stuck to their traditional model, and it ruins the experience that only GT could bring to the table.
All this said, I do take back my comment about less content than GT4 for over $60. I will say less content that GT3 for over $60. GT4 had an insane amount of content and as the first full game on the PS3 I do not expect that much from GT5 up front.
If Rock Band had come out with all hardware and those songs prepackaged at a cost of $500, how many people would actually have bought it?
When Rock Band 2 was announced I actually calmed a lot of angry friends down by pointing out that they are getting 80 songs at $0.75 a piece plus a much needed update to the interface.
And for the record, I'm definitely not saying you personally have insanely high expectations/demands, only that many appear to.
I was about to become defensive.

I think that I rarely complain about having my expectations not met.
And for the record, I would like to point out that I have yet to buy into episodic gaming where I wind up paying huge sums for the entire experience that could have been sold as a one time $20 game.
And someone will confront me with this, so here goes: Yes, this includes Watchmen: The End is Nigh. Unless I get a demo that proves to me it is worth two episodic segments I will be giving it the stink eye, despite my love for Watchmen.