There is a difference between "depreciate" and being undervalued. They're basically the same if you have to sell today, but if like in the stock market you can wait until the market rebounds, your car did not depreciate. Now before someone goes crazy on me, I DID NOT just equate buying a GN to buying stock. I am merely pointing out that a new car depreciates, goes down in value to never recover, collectible cars go up and down with the current market. When I had the chance to buy a GNX in 1987 with the MSO for 10K over sticker I thought the guy was nuts, little did I know that that car was not going to depreciate. Then again, the way I enjoy driving my cars, what would a GNX with about 100K miles be worth? It's all relative.
So, you think that when the market rebounds, your car will be worth 20K? How long do you think that will be? 5 or 10 years? If you take the lower 5 year number, You will now have a 100k+ motor and a 5 year old paint job with a nick here and there. 10 years? Then you might have to spend another 20K to get it good again.
I bought my car new for 18K. It is beautiful, and reliable, and 140k on the clock. Along the way there has been new paint, upholstery, and a few goodies. But, my car will never be worth 18K again, not without a cost of twice that.
Now, if you are talking about putting a car away and never driving it, so it will be in the same condition in the future, then that is another story.