I think the idea is not to buy the cars outright, but pay for credits as some sort of "boost" to your wallet.
I even think their idea was to have people spend monthly. They know many people play GT as their only game. What's $70 + $20 every month + whatever they're charging for PS Plus right now? If I did this, I'd get 24 million a year, plus whatever I grind from the races, which is slow, but if I was in my holiday period, I'd get to the free 20 million very easily. That's a lot of credits.
Obviously though, MTX have such a bad reputation that it backfired immensely.
But here's the catch. I think enough people will secretly spend tons of money on credits that Sony won't change things much. They'll throw us a bone, but I think the MTX are here to stay. If the UCD availability isn't fake (timed-only instead of stock-based) and the McLaren F1 goes out of sale quickly, we'll have our answer.
I've seen people say they spent money because they need to create content. Well... yes and no. Because their videos rely on their persona much more so than the content. As long as they're playing GT7, ACC, iRacing, F1 or whatever they play, they'll have viewers.
These things only exist because people give in.