With all 3 major titles having launch dates within a few weeks of each other, I think they all really have a lot of pressure on them to hit those dates. None of them can afford a slip now.
In some ways it was a smart move by Forza and GT to put that pressure on pCARS2, but in other ways, they have put that same pressure on themselves now too. Since SMS announced their launch date first, if Turn 10 or PD slip now it will look really bad on them.
I agree: at this point, they're all locked in, and it'd be a huge deal if they moved. Only one of the franchises has a history of never having its release dates slip, however...
It does create an interesting dynamic, one we've never really seen before. Yep, releasing as close to PCARS2 puts pressure on it from both first-party outfits on the consoles. If Jimmy Casual is looking for just one fairly realistic game this holiday season, knowing that they all release so close to one another, it might be easier for him to wait it out if GT is what he's after on PS4. He doesn't really have any other options on that console, outside of the original PCARS and AC. An XB1 player has a bunch of previous Forza titles in addition to those two.
PCARS1 was launched in a very different time on PS4. It was prior to GT Sport even being announced: if GT fans wanted a serious racer, that was the only choice unless they were staying on PS3. I wonder how many of the PS4 sales of the first game were because of this, and I wonder how many will be buying the second. It's quite a big jump in terms of features, so that bodes well for it.
Of course, as you mentioned, this proximity puts pressure on GT and Forza too. Being launched so recently after PCARS2 will drive home the comparisons. GT arguably has it even harder, since it comes after both (and FM7 is available on PC now). It has a smaller car list than even PCARS2 (and 1/4 the size of FM7's), but that list is also far less diverse. It has a massively smaller track list than either other game, and a fair chunk are fantasy circuits. It does have the name recognition, though: I wouldn't be surprised if GT Sport still sells more than either of the other games. But for the first time, I'm not 100% sure of that outcome... only about 90%.