I thought we were over this by now? I've been playing since day one, maybe not as much as I used to but still a massive number of hours have gone into GT5, and I still haven't got the Mk.IV or 330 P4, and they're the main incentive for me to keep playing on my main account. The sooner you get all the cars, the sooner you lose that incentive.
Let's not forget that if you have the 200% bonus you'll get 8 million every time there's a new set of drift and time trials out, the X2010 event would have paid out 10 million (though I doubt the events to come will be anywhere near that profitable) and we now get bonuses on everything, even B-Spec (maybe even remote racing, I haven't checked).
Finally, expecting PD to willingly make it even easier to shortcut the achievement of earning 20 million is a bit unrealistic, why would they do that even after offering huge rewards for early seasonals, triple rewards on later seasonals and now double payouts on everything? It's already far, far, far easier than it was in the pre-seasonal event days... I've done too many laps of Indy in my life to want to go back to that.
Edit: an auction house would be great, but would be far more worthwhile if we could properly customise how the cars look. Otherwise you're just selling something with maybe a rare paintjob and a tune that could be found here for free.