You might want to double check your figures here.. a $40k car that is 10 years old with 100,000 miles on it will command less than $14k.. try using KBB.com (Kelly Blue Book).
Here is an example.. a 1996 BMW 750il with 90,000 miles on it, in excellent condition, will go for about $14,230 from private party. A 1996 BMW 750il did not cost $40K new.. try closer to $80k to $90K.
Here’s a 2000 328i with 70000 miles, 10k miles/year: Price is $15k for good condition and $16k for excellent condition. 2000 328i
Here’s a 1999 328i with 80k miles, 10k miles/year: Price is $13k for good condition and $14k for excellent condition. 1999 328i
There’s a major price drop of $4k when you get a 1998 with 90k miles because it’s a different generation. So my example still stand, a 328i was around $40k in 1999, is now $14k after 80k miles. It’s still just an example, don’t take the number so literally. The point is that if you plan to keep a car for a long time, the cost of ownership is not that much difference between a new car and a used car. There’s also other benefits of getting newer cars, such as better safety, better power, better gas mileage, etc.
Even with your 328Ci Convertible example, the used one would cost you about $95/month over 10 years instead of the initial $116/month I mentioned. Would you rather spend another $20/month to drive that newer car? I would assume a car in the first 100k miles would have much less trouble than the car in the 2nd 100k miles. That’s when the major repair cost tend to rack up, like timing belts and etc. So if you own a new car for 200k miles, you would only pay for those big fixes once every 20 years while if you buy a used car w/ 100k miles, you have have to do it twice every 20 years. That would also factor into the price difference.
