Even though there are evidence of this being a "full" game, I still treat it as a prologue and the "evidence" to be just marketing trick/terms (honestly)....... then again i guess different people see things differently....
Well whether someone here, who has access to all the information available for GT Sport, thinks this is a full title or a prologue, is irrelevant to 99% of the potential buyers, as they won't be trawling the internet for info about the game.
I know anecdotes aren't evidence of anything, but bear with me here. I've had a few close mates tell me things along the lines of "Hey, you gonna get that new Gran Turismo?" or "can't wait 'till the new Gran Turismo comes out". Now, of all my friends, not a lot of them are gamers, and of those that are, not many are car nuts like myself, so only a few of my friends would ever consider buying racing sims, so only a few of my mates have spoken to me about GTS. One who asked if I was buying it was shocked when I said "nah, don't reckon mate", because he knows how much I'm into racing in general. He asked why, and I told him the game was essentially online only, and very light on content compared to any other GT game, and he said "what, really? Oh **** that then. I was gonna buy it, but it doesn't even sound like a Gran Turismo game. Glad I asked you".
Another mate who was telling me he was keen for it to come out, I asked why, and he didn't know a single thing about the game either. I filled him in on what I knew about it, and he too was instantly turned off the idea, and said "jeez, maybe I should get an X Box and just buy a wheel for Forza" (he hadn't bought a wheel for his PS4 yet, but was planning to). I've had a couple of other conversations similar to the two above, with other mates.
Now, these guys to me, represent the average GT buyer. They're not hard core sim fans, they're car guys who are also casual gamers. They don't follow the development of these games, or even look for info about them. They just see "new GT game coming soon" and think, "yeah, I've always liked GT games, I might pick that one up when it comes out". They were both completely turned off buying GTS when I told them what it actually was, so this is why I'm willing to bet there will be a large majority who will simply buy it because of the name, without knowing anything about it, and then be immediately disappointed because it's not a typical GT game.
In outright sales numbers GTS will be successful, but in making fans of the franchise happy, I don't think it'll do too well.