I've been with the GT series since GT2, and with 2,3,and 4 I never cheated. I collected every car as well as every RM version of cars on GT2 legally, had multiple memory cards to store them on to get around the garage limit. GT2 the career mode was already fairly long with a lot of prize cars, after that you could earn about 300,000 in 10 min or so, the most expensive car was 2,000,000 I believe so wasn't that hard to collect everything. GT3 had a lot less cars, and fairly good payout so no problem there. GT4, a quite long career mode, plenty of prize cars, and you could earn I think it was 785,000 in about 25 to 30 min or something, most expensive cars I think were 3,500,000. Once again not a far stretch to collect all cars, although I never did since some cars for the first time I believe could only be gotten through certain events like special events or license test, which was an annoyance as I really hated that 240sec wait between attempts on the last special event, however I came close to collecting them all. GT5, what can I say, really shot A-spec mode, B-Spec got way too much attention, the racers are all double length of A-spec events to B-Spec is really twice as long, not that many prize cars to be gotten specially considering few will ever attempt the 24 hour races. I run the 24 hour events in GT4, with B-spec speedup I could do it in about 10 hours, not that bad for a fairly cool running PS2 and wouldn't cost that much to replace. Running a PS3 for 24 hours, not happening for me, specially considering I have at times had random power outs in the location. Add to that car prices at 20 mil, best credit earning at around 117,000 per 4 minutes, so around 1.7 mil an hour, now it's back to 98,000 per 4 min so around 1.2 mil an hour again. You are looking at 60 hours or so grinding just to get a few of the most expensive cars, and ones like the GT40 you will have to get lucky for it to show in the UCD otherwise you are sitting on that 20 mil.
Breaking it down, early flaws of the game, exp too low, credit payout too low, lack of events, a wall once you get to level 25 or so and a huge I mean 100x greater than any GT game before of grinding to try to collect all cars in the fair way. Even with duping which GT5 makes the first of the games I've ever cheated with, it is still a huge task to collect all cars, few have done it at this point, as first you need to find the cars in UCD or find someone with them to trade, then you still can only do it once a day unless you use multiple accounts then it still would only be a few a day probably. I started trading a couple months ago, and useing the save trick a couple weeks after it was allowed and I still only have gotten to around 450 cars now, aiming to get most of the big priced ones out of the way first. I found this fun though, it actually got me back into playing GT5, I actually quit playing a little while before backing up saves was allowed because I couldn't see myself grinding for 300 hours or more just to buy the 100 most expensive cars. So trading became fun, it was like using a large number of ucd to get the cars you want, getting a couple new cars for your collection a day. Some cars like the 370Z academy, you may miss entirely, if you weren't lucky enough to get it from a level ticket then your only hope is to get it through trade or make new ID's and repeat till you get it, without duping who would trade one, so only option 2 would be logical, some fun there.
So what do we have now, well the most rare cars are off the market, so it's back to flipping through the ucd over and over till you finally find the special car your missing. Add to that since you can't trade those high dollar cars you can't buy the really expensive cars without cost anymore, true a cheat but this means the grind is back, if I find a 2J in the UCD, 14 mil spent, 8 hours of grinding to recover. So collecting all cars just got a good 20x harder and it was already hard even with cheating.
GT5 is just a screwed up game, they keep changing everything so you can't build any kind of strategy to playing the game in an enjoyable way. If they want to fix the game here is the steps I think they should take.
-Leave exp as it is, the recent increase feels better, gives less reason to fill like you are exploiting seasonal events, you can choose new events easier since completing one may level you up enough to choose several, "good new change"
-Increase money payout or make prize cars sellable and repeatable like the old GT games, money payout is way too slow to ever try to collect all cars which is what many like to do. The payout is fine early levels but it should increase a lot more as you get to high level events, more like 750,000 for each FGT event with a 2 mill bonus or something would work better for a high level event.
-Increase standard events, game is still way too short, seasonals are fun but not a replacement since they rotate you can't go back and play the best ones.
-Restrict level on seasonals, it's not right how playing one seasonal can jump you from level 2 to level 12, makes early game a big skip, the payout and exp is good for high level players looking for new racers but unbalances low levels. Level restict them and make them repeatable may make them more fun and cure game balance issues.
-Open back up trading of all cars or at least make the most wanted rares cars permanent members of the online dealership. People won't have reason to need to trade for cars as much if they can buy them.
-Either create a save system for the longest endurance racers or bring back b-spec driver subing with acceleration. Real life drivers can run up to 4 hour stints, and maximum of 8 hours total. So any driver racing for over 8 hours is unrealistic to begin with. Also in reality after their stints are done many drivers take a nap so having to watch your b-spec driver run the full race isn't really realistic either, so speeding through it really isn't much different than sleeping through it and it's more green, less energy used from PS3's running for little good reason.
If the above is done, then people will be able to get money at a good rate and with more events the grind will seem to nearly vanish. Rare cars can be bought with the money people earn so people will likely be buying cars fairly. Game progress will be much more normal since seasonals won't upset it so much and the current exp rate flows pretty well. Since people can get cars in a more reasonable fashion then the constant trading of the same cars using exploits won't really be needed so trading will turn back into some more normal with tuned cars, rarer ucd street cars, or alternate colors of cars will become more the normal for trading. Top that with a more realistic yet still challenging take on endurance races and you might see more people actually complete the game fully as most now probably do everything but the longest endurance races.