Am I correct in assuming that you had plenty of time for career grinding when playing GT4 to afford what you wanted? Payouts were a lot worse back then, plus there were no in-game cash boosts available such as the seasonal event, or even the micro-transactions; sticky subject, I know - but it is there for the busy person to take advantage of. Or did you not worry about it and got on with things because you wanted to.
Payouts were worse but there were prize cars to even it out somewhat.
I'm still not going to claim that the GT4 economy was great, all of the GT games have had this problem to a greater or lesser extent. The main difference between the early games and the current ones is the size of the gap between the cheapest cars and the most expensive.
Let's use GT4 as an example. The most expensive cars there are 4.5 million. Let's assume that the payouts are about the same, when taking into account prize cars and all the rest of it. Let's also assume that the distribution of car prices is relatively the same, only a few at the very highest price down to a lot at the low end. These assumptions likely aren't correct, but they're probably not far off.
In both games, the player likely has a bunch of relatively cheap cars which are easy to afford in either game. No problem there. The player likely has a few very expensive cars that they have their eye on (or all of them, since the expensive cars tend to be some of the most desirable), and those cars are much less affordable in GT6.
The problem is the scale of the gap between "cheap" and "expensive". It's a design problem that makes it very difficult to balance. It means that the late game economy has two possible modes, both of them with drawbacks.
Firstly, you can give out money at a rate that makes the most expensive cars at least reasonably affordable. This makes all lesser cars trivial purchases, and diminishes their value as possessions.
Secondly, you can keep the payouts low to try and maintain some value in the majority of the car list, but it makes buying the most expensive (and probably most desirable cars) difficult, and makes owning more than one or two pretty difficult for a lot of people.
There are ways around this, such as making cars accessable in Arcade mode so that GT Life isn't totally gating people who just want to drive. Ultimately it's a design flaw in that the way the game deals out cars is only suited to those people who enjoy sinking significant amounts of time into acquiring an electronic possession. For those who don't have that very MMO-ish trigger, it's simply an irritant that gets in the way of enjoying the good things about the game.
If you're actually interested in how I played GT4, I had a lot of the same problems with it that I have with GT6. Game design-wise, it had a lot of the same problems that the series has had more or less since inception. At the time of GT1 and 2 that sort of design was pretty normal, even above average for it's time. It hasn't really moved on since, and as such there's a bit of a perception of it getting worse over the years, when really it's just standing still or maybe moving very slowly. But relative to the expectations of an increasingly savvy gaming public, it's not adapting.
Save editors would be a lot less appealing if some of the most attractive things they offered were available in the base game. There's always going to be those making 40khp Civics, but those people are always going to get their jollies by trying to break stuff. The people that PD should be taking notice of are the ones who are using it for a little extra cash to enjoy GT6's magnificent car list, the people who are making realistic engine swaps and chassis mods and the like. These are things that players enjoy, and might be appreciated as actual features in the base game.