Nothing has actual value until you sell it. Sure you could stop putting money into your car and go and spend that and more money on another car, but then you are starting all over again. Cars have never been investments, so why people get caught up in the repair cost to value thing, I don't know. The way I look at it, if your car is "worth" $3,000 and you spend $3,500 to bring it back up to speed and be as reliable as a new car, then it's worth what a new Benz would cost. I know this goes against the grain of conventional thinking, but really, when it comes down to it we are talking about transportation. ABS, ESP, GPS, Nav. Sys. Statelite radio, etc. are not needed to get from point A to B.
Take what you paid for the car plus the maintenance/repairs, divide that be the number of months you have owned it, and you'll see what on average you have spent for that car. It will certainly be less than a car payment, especially one on a new Benz.
It's the industry that wants to to toss aside the "old beater" and get a new one. Not that I am an enviromentalist, but you could think of it this way, you are keeping one less car out of some junk yard/land fill

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member