DLC is nice, but really, if it gets too much, we really have to ask ourselves what's better for us in the long run. Do we really want to spend upwards of 50 plus dollars just for simple content add-ons like cars and tracks in the next year or so (till whenever GT6 releases), rather than just have them all together packaged in a new game?
I doubt there will be any more major changes such that of Spec 3, because really, any major changes to the game PD would probably want to save for GT6, plus on top of a lot of other things that requires a reconstruction to GT5's framework such as a completely new GT Life experience, which is what everyone is pretty much anticipating with hopes of an event generator for GT5.
And remember, especially with this bit of news, it's pretty clear PD has quiet a bit of content at their disposal that we don't know about. However, it doesn't mean they're just gonna give it all away because as said before, it'll take away some marketing appeal if they do. What that also means is that they'll be forced to model more cars (and tracks) than intended for the reason that there will be more newer things that we can look forward to, which in turn will push off GT's release date even more. Whether you agree or not, releasing a game that has content that's already been revealed and playable isn't as appealing as you think. Otherwise, why would we demand DLCs so much?
There is no reason to think that GT6 is a must for the PS4. It can, but it doesn't have to.