So what, if everyone owns everything it's not a big deal and that won't happen anyway since there are cars priced at 20 million and loads of people won't even own one of them, even at this point in the game, so soon enough will actually be never for most people. The only people who own all of them and most (maybe all) cars in the game are the ones who spend endless time grinding for credits or even setting up a system that runs for them, that shows just how silly this system is.
This isn't a marketplace dependent game, what someone else owns does not effect your game in the slightest once you own the most Op Gr1/2/3/4 cars and they are easy enough to get. It's also not real life so there is no need to maintain a stable economy on that count either. Maybe if this was the last GT ever to be released and new content would come forever then it would make sense but this isn't the case. It's not like PD benefit from this system either, if you could buy the 20 million cars for real money or you could shortcut the grinding by buying credits for real money, then again I could understand why it was like this but as it is you can only buy the more easily attainable (both by credits and by winning) cars for actual money anyway so it seems kind of pointless.
The only thing that tops this for silliness is the levelling up system, this wasn't even needed and to get to level 50, playing what I'd would describe as normal amounts over a week, it would take a number of years worth of play and for what, a meaningless (to me anyway) PSN trophy. It's no wonder people looked for shortcuts to achieve this and these shortcuts helped spoil the game at multiple points of it's lifetime too.