I don't understand why anyone tries to defend the low payouts/20M credit cars. I've put over 180 days and over 200 hours into GT Sport, including 94 Sport mode victories, every launch day event and most of GT League, and I've only won about 35M credits in total. So not even enough for two of the 20M cars, not to mention the others at 10-15M. Does that really sound fair to anyone? It's ridiculous.
You should absolutely NEVER have to grind some offline campaign mode race over and over to get a decent amount of credits. Performing in Sport mode races, whether it's winning, podiums, top 6, top 10, etc. should 100% provide more credits than beating the same uncompetitive AI over and over. Complete ass backwards thinking from PD as usual.