A convenient attitude to take after posting this:
"If Forza 6 isn't ahead, considering it was released almost 2 years later, there's something wrong.
What else needs to be said?"
...and then arguing it for a half dozen posts.
That FM6 was released two years later has little bearing on its car list anyway: the only thing that really changes is which cars are included that debuted after GT6's release date. Even that isn't a hard line these days thanks to DLC, though PD largely focusing on the VGT program instead of production models does affect it. The majority of FM6's post-2013 models are race cars, anyway. The Group 5 vehicles, pre-war ones, vintage F1... None of those are really affected by the release date.
Besides, as
@Samus hinted, FM4 had, in a few ways, an even better car list, even today, than FM6 has.