The analysis is not over simplified. You're right that you don't make $225, 000 at the start, but your start up time to make that much is the same number of years whether or not you do a MD or MD-PhD, i.e. it takes about 3-4 years of practice before achieving that level of income. Therefore, by doing a PhD you're simply taking 4 years out of your career where you'll be making that level of income before retirement.
Interest on student loans have historically been low. The average medical student debt at Hopkins was $150,000. If you pay this over 10 years, it works out to be about $250,000. Financially, you're better off doing an MD program in regards to the total life-time financial reward. You'll still be ahead $500,000 over the life-time of your career. If you're worried about not having money for a car, then borrow up front and pay it back later. You'll still be ahead after you start making money.
I agree with you that people need more money up front than later on. This is why the military is an outstanding option.
However, if you're doing a MD-PhD program for JUST THE MONEY, then don't even think about it. You'll have great difficulty completing the PhD program. The PhD program is more difficult than medical school. I've known many MD-PhD candidates, graduates, and professors, and I've also experienced the PhD program first hand. You must love research and want to pursue academics; otherwise, you're going to be miserable for 8 years, particularly when your friends become your attending physicians!
The point is clear. Go into the MSTP program if you love research and truly want to tackle the rigors of original scientific research. The scholarship is only icing on the cake. This is a mute point because you'll never make it through the 6-8 interviews for the MD-PhD program if you really did not like research. At some programs, you have a committee interview with 5-8 faculty too.
Personally, I don't think it's a waste of time to do a Ph.D. regardless of whether people want to have a career doing research in the end. PhD in and of itself is a training degree and it exersizes your mind and independence, and I'd imagine that is its purpose rather than preparing you for a particular pathway of career.
You're incorrect about the MSTP program being not designed for those who want to specifically enter academic medicine. This program is funded and started by the NIH for that sole purpose. NIH wanted to train medical scientists to bridge the gap between clinical medicine and biomedical research; hence, graduates were expected to enter academic careers. Not all do, but 95% of graduates eventually enter academic careers doing research, teaching, and caring for patients.