Right, all this incessant "the economy's not broken, it's Gran Turismo, you're just moaning and groaning" and "the economy is broken because the game forces you to grind (for what you want)" has made me break out GTPEDIA. I hope you're happy.
Ignoring prize cars, which are to a greater or lesser degree "set" and not under the direct influence of the user, and assuming all of the one-shot events have already been completed for the prize money available, here's how many races you have to do to get the most expensive car in the game in the most efficient way possible:
GT1:
Car - Any Special Car: 500,000cr
Race - Hard Tuned Car: 8 races (~1hr); 50,000cr race win (x8), 100,000cr championship bonus (x1)
Rate - 0.5m cr/hr
GT2
Car - Various: 2,000,000cr
Race - GT500 Car Championship: 14 races (~1.5hr); 50,000cr race win (x14), 300,000cr championship bonus (x2), sell prize car (x2)
Rate - 1.35m cr/hr
GT3
Car - P001/F094/H: 3,500,000cr
Race - Special Stage Route 11 Endurance - 5 races (~7.5hr); 550,000cr race win (x5), sell prize car (x5)
Rate - 0.5m cr/hr
GT4
Car - Various: 4,500,000cr
Race - Rally de Capri (Easy): 32 races (~2.66hr); 5,000cr race win (x32), sell prize car (x16)
Rate - 1.69m cr/hr
GT5
Car - Various: 20,000,000cr
*Race - Extreme/Dream Car/B-Spec: 315 races (~23.25hr); 22,700cr race win (x315), 206,000cr championship bonus (x63)
*Rate - 0.87m cr/hr
GT6
Car - Various: 20,000,000cr
*Race - Red Bull X2014 Standard Championship: 90 races (~5.8hr); ~128,000cr race win (x90), 500,000cr championship bonus (x18)
*Rate - 3.55m cr/hr
GT Sport
Car - Various: 20,000,000cr
**Race - Nostalgic 1979/La Sarthe: 84 races (~9.9hr); 240,000cr race win with Clean Race bonus (x84)
**Rate - 2.04m cr/hr
So, as usual, the truth is somewhere in the middle. Yes, it takes longer than in GT6 to earn the money required if you want one of the highest value cars, but it's actually still possible to earn money at a much higher rate than in most previous GT games. But aside from GT5, it takes the longest amount of time to actually buy one of those highest value cars, because they are much more expensive than in previous GT games - and GT5 was actually just about as quick when the servers were still on.
The net result is that GT Sport takes longer than any other active GT game to buy the highest value cars, even if it's in the top three for the rate at which you can earn money.
That's why it seems like a grind to some people but not to others - money can come quick (four times faster than GT1), but you need way, way more of it to buy the most expensive cars (forty times more expensive than GT1).
But all that aside, the fastest ways to earn money are offline in a game that didn't originally even have that offline mode because the focus was online esports. If PD want to encourage people into online, a zero on the end of the payouts will help, and having it tied to your SR will help even more...
*Both GT5 and GT6 had higher-paying events when the servers were still live, plus a login bonus.
**Requires high value car to buy in to the race; next quickest is 1.8m cr/hr, for 54 races/10.6hr.