Paths change, resource allocations change, market conditions change, nothing is set in stone. If sales are anything close to what is bandied about, in the 3-4 million unit range and slowed to a crawl, that's a dramatic drop relative to GT5 at this point. GT5 was double that at this point. If that is true, I can see them deciding to only work on the minimum possible for GT6 and all other resources to GT7. Even rats abandon a sinking ship.
Not sure where this "pure profit" comes from because if they are true to course, any DLC is going to be free, so you not only have the extra cost of implementing it into the game, you lose the marketing opportunity of debuting the new track on the new system. It's actually pure loss at that point unless a new track is going to push a significant number of units out the door which I seriously doubt. Course Creator, Community Features etc. would be part of the bare minimum I mentioned.
IMO the entirety of the engine/car sound package for GT5/6 is the worst in all sim driving/gaming. There may be some subtle nuances in there that you and others appreciate and enjoy from a technical standpoint but for me and it appears many others, GT is the least exciting driving experience from an aural standpoint by quite a large measure. I'm not a sound engineer, I can't explain beyond some basics why I feel that way, but my gut and my adrenaline level doesn't lie to me. I'm sure you disagree and that's fine but IMO GT has a long, long way to go to even be mentioned in the same conversation as some of the better sim/driving games.