Or why can't we at least accumulate 1M per day if we don't trade, and trade a 20M car after 20 days for example? I think this could be a good compromise.
I think this cumulative trade idea is a great compromise!
Ultimately, people will need to realize that PD did this for 2 reasons.
1. They didn't want people duping. Whether you think it's cheating or not shouldn't matter, PD thought it was a problem. and...
2. People selling cars on ebay for real money. Not only is that a copyright issue, but it's just plain wrong. Of course... the wrong bit is just my opinion, but copyright problems are law (based on country of course).
So the only "fix" to this problem will likely need to keep those in mind.
I don't see PD keeping the limit on permanently, because it will make people sour about the game, and lets face it... when GT1 came out, it was ground breaking, but the competition wasn't exactly the greatest. Now that GT5 is out, people have other options.
That said, I think that cumulative trade limit would be great! It would significantly increase the effort required to dupe cars, and it would still allow freedom for trading. It would also help to add to the longevity of the game.
I expect that PD will probably lift the cap after some time because they'll assume that only the "hardcore" fans will have stuck with the game for so long, and all of the people that were just duping will have moved on.