They didn't. The "Stealths" appeared on game days 694-700, by which time it was possible to have completed 90% of the game.
They couldn't. Microtransactions (cash-for-credits) didn't exist, and cars over 10,000,000cr (initially 2,000,000cr, but this was raised in March 2019) couldn't be purchased in the PS Store.
Assuming that you either knew the Stealths were coming and had saved money, or that you played the game without ever backing out of events and advancing the calendar more than an optimized route. Either way, cars in the Used Car dealer were not inherently affordable on Day 1 of the game, nor were the Stealths the only large sum cars, and being able to buy all the available Stealths required a large sum of money since they put them all on the same days.
And how many people were forced to spend real money for 10mil credit cars? None. 10 mil, 20 mil, it makes no difference to the point that the existence of microtransactions does not force someone to use them just to get what they want.
The consumer pays £70+ for the game. That should be enough entitlement to gain access to any car on the game or be able to raise 'good enough funds' in a certain amount of time to attain such cars. There is no such as 'deserving' when its merely a virtual car in game that is worthless and these certain cars was easily accessible in past GT titles.
No, sorry, you're not entitled to that. A game with a progression system doesn't have to hand you anything in the time scale that you demand. Guess what, the cars weren't free in previous GT games either, you had to earn credits for them too. Having that earning be easier or harder does not negate that you still had to do the exact same thing, and could not inherently gain easy access to every car you wanted.
The 20 mil credit cars in GTS weren't on sale for a week before disappearing into the shadow realm. Besides, the way Polyphony did all this FOMO nonsense/credit income nerf is the scummy part, not the fact that people can't get the car in one day.
Except as has been pointed out ad naseum, the used car dealers have worked in this way before. Just because you were eventually able to work out how to game the system to get the UCD to give you the options you wanted does not change the fact that PD has offered you cars then taken them back away and made you wait, either through non-playing gaming the system or playing the game naturally.
If you're gullible enough to believe that a game should have had a small rally event being a huge payout that you were expected to grind, then I have a plot of land to sell you. They fixed a mistake (as well as several others in the update), they didn't decide to nerf things. If they wanted to make you grind the way to these cars, they would have had the payouts be lower from the start. PD knows how to do math.