If you play the game for long enough you'll have all the cars, maybe in more than one exemplary each.
And if you don't want to play the game for long, what's the point in collecting all the cars then?
Long enough might vary per person/opinion. For the casual player over 100 hours is far too long already. But most importantly it's about how you gonna play the game.
I've already posted in another thread, but my favorite thing to do in GT is just taking a random car (prefer the road and/or old cars, though) and driving it around a few tracks I enjoy the most.
With how much it pays out now for those custom races even if I'd play the game daily for years I'd never get cars I enjoy driving. You get what, like 30-40k/hour at best? That would be over 12-13k hours of those to get ~410mil needed to buy all the cars, make it "just" 8-9k hours if you're skipping most of GR3/4 and VGTs, since LCD cars cost the most.
Sure, Sardegna is still there, until/unless PD decided to make some "awesome changes" again, that's 25 minutes per race, 50m for two and that would be enough to get all cars within a year of daily playing (not counting future cars additions, which will make it even longer).
Problem is you spend hour of your time on grinding, just like before, it's just even slower. And you have less time to do other things in game. If you're a die-hard fan of GT7, then sure, you might get there anyway. For anyone with other hobbies and interests in games... nope, that's far too slow.
What would an average player do, choose to play and complete another game within 15-25 hours (average for the non RPG/JRPG/Open world games) or grind away for one fancy car in GT7? It's especially relevant for those with very limited playtime, which can be less than 10 hours a week.
GT7 really needs a natural way to acquire credits without repeating same thing over and over again, be it the exploit way or not. If PD wants to keep players around at least, and since it's GaaS/live service game, I'm pretty sure they do.
P.S.
Hardcore fans that enjoy just about everything in said game is never a good indicator of any game's content and longevity. For instance I have nearly 4k hours in Dark Souls 3 accross PS4 and PC versions, another 3k in MHW on PS alone. Doesn't mean they have as much content to keep an average player playing for years.
Same goes for the GT7, except content here is partially locked behind the credits and/or microtransactions wall. The wall most will hit and bounce off.