First and foremost, if it were up to me, the Hagerty dealership would not exist, at least not in its current form. Part of my issue is that only a small selection of these cars are available at any one time, and if you miss a car, you don't know when it'll be available again, which creates its own significant source of frustration. Having a dedicated spot for the rare cars is fine, but the revolving-door mechanic...
Beyond that, I would just do more to give players options to make money. I'd let players have the ability to sell cars and parts for extra cash with no negative impact to collector level (I'd probably get rid of the collector level too, honestly), as well as re-introduce GT5s car-trading system, but with a couple tweaks. The idea of letting people "rent" players cars for online races could be kinda neat, and has been swimming around in my head for a number of years. It would be a way to create a source of passive income, would be based on results (meaning that, depending on the event, upgrading and tuning cars would still be important for success), and it'd be just quirky enough for PD's tastes. With all that, people can still make decent cash, high-end cars aren't as easy to miss, and the Hagerty dealership can keep its dynamic pricing gimmick.
I would still make it so that there were more events that gave out prize credits similar to Fisherman's Ranch, or at least scaled appropriately with their length and difficulty level (I have yet to play GT7, so I don't know each individual events credit payout). As
@Samus has already said, there really needs to be more options for higher-paying events more than anything else, so that people aren't tied to playing Fishermans Ranch over and over again for ludicrous amounts of time. Online and custom races would also get a re-vamp so that people who don't want to focus all their time on the career still had an avenue to get money and earn
some of the cars in the game.
In any case, if I absolutely had to put down a hour "commitment" for the XJ13, I'd say maybeee 3-4 hours or so if you
really try to maximize profits, no more than maybe 5-6 hours reasonably? The actual $20 Million cars taking 8 hours or so of gameplay
at absolute most is adequate, due to their higher price point, and hopefully smaller number. More than anything in regards to those cars, I would make it so that whether you have the cars or not doesn't majorly effect progression, so that the people who want them are the people who are truly dedicated to them, without hurting any other players that don't really care.
Of course, that's all moot because I don't know how to effectively tear-down and re-design a game's economy, nor (ideally) would I be doing such a task on my own.
So now we're calling people with genuine addiction problems "fools?" Stay classy, chief.
IIRC, GT Sport actually had this system, where you could just straight-up buy cars for $5 at most. While I'm still not crazy about this system, I'd be more ok with that than what's present, because you at least fully knew what you'd be getting.
With all due respect, that is 100% a you problem. There are so many other ways to make getting those cars feel special, without having a terrible grind that promotes MTX purchases. An example would be making those cars prize cars in higher-difficulty events, something that GT has historically done in the past (GT4-R89C-Gang rise up), or having some cars be "milestone" achievements, such as having a certain percentage of the game complete, or by completing all the races with a certain license rating. A car-trading system like GT5 could also work, except you have to give a car(s) similar in rarity or value to what you're trying to get. That creates a more involved system for the player, because you're potentially giving up a car(s) you may be fond of to get something you really want, which also creates a better sense of community among players.
I'd rather spend a couple hundred hours actually enjoying the game, doing online races, tuning cars, etc., rather than trying to grind for 3 of the more expensive cars in the game, all of which are tied to my progression.
I would've rather had PD spend more of their initial development time making the game as whole enjoyable, and giving me a reason to come back because of that (or, more accurately, buy GT7 and a PS5 in the first place), rather than try to figure out how to best get me and other players to further line their and Sony's pockets after spending $600+ just to
use their products.