As long as those cars continue to exist, they will continue to be justified as being the obvious stick with which to beat Turn 10 because of things Turn 10 said themselves. Those models weren't good when they were first made and Turn 10 has not done anything with them even when they skip being in a game (like the SW20 did) to fix them. Gamers may have a pretty short term memory on most things; but being actively
lied to to temper criticism isn't usually one of them. See: Variations of "Game Freak lied" trending on Twitter worldwide when Sword and Shield came out over what was pretty much the same thing (ie.
developer defends backlash against tons of cut content by saying it was redone from scratch to higher quality standards,
people get the game and find that it's identical to the previous game and that the developer was lying the entire time).
I also reject the insinuation that outside of those 5 (or whatever the full number may be) truly bastard children that Forza's cars are all accurate as a point of comparison to PD's output. Turn 10's track record doesn't support that statement enough to make it without scrutiny, and I can think of one car off the top of my head (the C4) that they seemingly completely remodeled as a different trim level and (despite being DLC) screwed it up again anyway in similar ways to how the 360-era model was poor (which again makes me wonder whether they
actually remodeled it or just touched it up into a different car and sold it as DLC and thus justified all the backlash against FM5 about that exact issue; though I haven't had Forza 4 in years so I can't directly compare them myself). You also still have cars that in real life were identical in the underlying body shell that clearly had different bases when they were recreated in the game as different models (like the GM A-Platform cars). The painting of Turn 10's uneven asset quality as a relic of the days before they claimed to have started over with FM5 has not been borne out in reality.
I know it'd be great to have 100% of the cars in perfect quality, and a lot of people like to say "quality over quantity", but truthfully when it's only a handful of cars out of almost 800 that are truly inaccurate, isn't that pretty much quality and quantity?
In the context of the OP's questions, no it's not. "Gran Turismo has just over 300, how is it possible, what's the secret that PD need to be in on so that GT can match the Forza prowess?"
It's possible for Forza to have so many more cars because PD
actually started from scratch (or at least did a far more convincing job than Turn 10 did) for the previous console generation and have always built their assets to a higher quality standard/closer attention to detail than Turn 10 anyway; and Turn 10
said they did to stop the backlash against Forza 5 but actually just ported stuff from the 360 and modified it (or
didn't, for a handful of them) while also making new stuff as time went on.
Put another way, the actual answer to OP's question is that PD's models look better, have always looked better and seemingly take longer to make as a result; and they actually seemed to have ripped the bandage off and started from scratch with the PS4/Xbone console generation instead of just paying lip service towards doing so when people complained at them. That isn't to say that there aren't examples of PD models that have gaffes in them (like the Premium C5 in the PS3 games, or that one that was discovered to have trademarks for a scale model on the underside of it in GT5) or that the Standard Cars thing wasn't an absolute joke; but the argument that Turn 10 don't still have problems balancing on the quality/quantity tightrope (or that their struggles with it also haven't infected the game structure) is facetious.
The player needs to make the determination themselves if whatever the car count is in GT7 is too low to justify the higher quality in comparison to the probably higher number but also almost certainly more fast and loose attention to detail in Forza 8.
