I think the only way to solve this, is by increasing the amount of credits earned per race. Or adding easier ways to earn a large amount of credits.
Let's see the different methods of earning credits in past GTs
Gran Turismo 4: You can make easy credits by winning the
Deutsche Touring Car Meisterschaft in the European Events Hall. Everytime you win
that championship, you will be rewarded with a
Mercedes-Benz CLK-GTR Race Car '98 and you can sell it for
700,000 credits. The best thing about this method is that you can use B-Spec mode and speed up the race, meaning that you can focus on other things while grinding. The estimated amount of time, is
25-30 minutes, meaning that you could make over
1,600,000+ per hour, even more.
It takes 3 hours to get the most expensive cars (4,500,000 Cr).
Gran Turismo 5: There was a seasonal event called
750PP Real Circuit Tour (La Sarthe 2009), where you race against a Jaguar XJR-9, and the amount of credits earned per race would be higher based on the PP of your car. If you car had less PP you could get more credits. For example, with a Chaparral 2J you can earn over
1,700,000 credits (with 200% bonus) in
10+ minutes, meaning that you could make over
8,500,000 credits per hour. It takes 2.5 hours to get the most expensive cars (20,000,000 Cr).
Gran Turismo 6: You can make easy money by winning the
Red Bull X2014 Standard Championship. I you get 1st place on each race, it gives an amount of 642,000 credits, and other 500,000 if you win the championship (1,142,000 credits in total). With the 5-day 200% bonus, we can duplicate the amount of credits earned by 2x, which gives us
2,284,000 in
25 minutes, meaning that you could make over
4,568,000 in 50 minutes. It takes 3.4 hours to get the most expensive cars (20,000,000 Cr).
Now compare this to
Gran Turismo Sport, according to
this video, we can make
2,200,000 credits per hour. Meaning that
it takes 9 hours to get the most expensive cars (20,000,000 Cr).
That is TOO much grinding compared to previous GT games. I think that 3 hours is a more acceptable amount of time when it comes to getting an expensive car, compared to the 9 hours from GT Sport.