If Porsche has an high demand from the GT player base, then it's likely that PD seeks Porsche. Only then they will see if a possible deal is worth it. If they are aware of the changes, we don't know, only when someone gets an interview or manages to talk to someone from PD about that subject, we will see.
Here's some other high-demand cars.
It doesn't change anything.
The thing is, there's no logical reason for them to not want their cars in GT. Fictional cars don't seem like a reason, since they won't hurt their sales nor image. One might say "Oh Porsche won't like seeing their 919 loose to a VGT car", that shouldn't be a problem, wouldn't it be the exact same "problem" for them to loose against an older LMP1? And some will say "that's why we need all the LMP1 from the same year", but then again, the class would be limited to 3 cars (4 at max).
Any other possible reasons?
That's actually quite a fine logical reason. You may not like it, but that's another matter. The idea that Porsche's two-time-winning Le Mans racer could be rendered uncompetitive against a bunch of fantasy cars could very well be enough for them to not want to play ball. Or maybe it won't bother them. But we don't know — nor do we know which situation is more likely.
Sure, they could want a bigger representation of their older cars, but we don't know what are the planes for the future of the game. Will the game feature classes of classic road/race cars? For now, we don't know, but it is likely since they keep giving the prizes in the Concourse d'Elegance and they had already moddeled some pretty exclusive cars. Who knows if after they complete these 5 main classes/groups, they will add some more.
The more games they have their cars in, the wider they can spread the range of new and old cars through those games. GTS would not have crossovers (atleast now), but has space for Caymans, Boxters, Panameras, "base" 911s (non-GT3/GT3 RS/Turbo/R), and anything that usually gets skipped in other games. And of course the high-end sports cars/supercars.
GT Sport, in its current form, can't include "anything that usually gets skipped in other games" though. As I mentioned, it caters to new cars and new cars only. Maybe that will change, but from everything we know about the game currently, that is its singular focus.
Doesn't make AC the better game, compared to the others, because Porsche chose them, or the others "less appealling" though.
It's a good thing I didn't say AC is the better game for it then.
It does make the other franchises less appealing, though: if they were more so, Porsche would've approached them. Porsche sought out Kunos. It was their choice. If they wanted to have PD, T10, or even the team behind iRacing, they would have approached them instead. Now, it's possible they approached any or all of them before Kunos, and all of them said "no", but that seems unlikely (especially in T10's case, since they've had limited access to Porsche for a while now).
Porsche wouldn't benefit much from doing a LMP1 type VGT. They already have modern LMP1 cars, why would they bother designing something similar?
Why would PD model two different models for Gr4 and Gr.N? A Cayman makes more sense in both cases, even a 911 makes more sense. And why just 4 cars? Not only 4 cars, but the least "important" cars from their whole range. If they pay for aa Porsche license, they should get the good stuff (918, 911 R/Turbo/GT3, Cayman/GT4, Panamera Turbo, 919 Hybrid and as you said 911 GT3 R/RSR).
Chevrolet has a modern Corvette race car. Polyphony modelled their own Gr.3 rendition instead.
Ford has a modern GT3-class race car (the GT). Polyphony modelled a Gr.3 Mustang instead.
Peugeot has a modern rally car (one Kaz has even hinted at including years ago). Polyphony modelled a Gr.B RCZ instead.
PD modelled two different BMW's for Gr.3 (three, technically). They've modelled two different Fords for Gr.B. Two Merc's for Gr.3 (maybe three, depending on where the VGT ends up).
What I'm trying to say is, there's no real rhyme or reason with the cars already included in the game, so it's pointless trying to apply logic to what
might be added.