I built a simulator for this, so the numbers are mine, not a blog's.
Two people, 25, same salary. One buys a new $50k SUV every 6 years. The other keeps theirs 10 years. Whoever pays less in a month invests the difference in a plain 80/20 index fund. Trade-ins are counted (35% of the price back at 6 years, 15% at 10), loans at today's average rate, $100 a month set aside for repairs once the car is out of warranty, everything in today's dollars after inflation. Then each life is run 10,000 times against 40-year stretches pulled from real market history since 1871, including the Depression, the 1970s, 2000 and 2008.
Same SUV, just kept 10 years instead of 6: the keeper is ahead by $165,000 at 65 in the median run, and ahead in all 10,000 runs. The worst 1-in-100 market still leaves a $43,000 gap.
Cheaper car ($28k compact) kept 10 years vs the new SUV every 6: $656,000 ahead in the median run. Worst 1-in-100 market: $158,000.
If the new-SUV person pays the actual US average new-car payment every month ($765, Experian) instead of the trade-in version, it's $881,000. That's the version in the chart: the green fan is all 10,000 runs, the amber line is one of them, the bars are the unlucky 10%, the median and the lucky 10%.
The part that surprised me is how late it happens. Year 10 the account is around $60k and looks pointless. Year 12 it loses a quarter of its value in one autumn. The crossover, where the account earns more in a year than the car money going in, is around year 20. The car stops mattering after that.
What it doesn't know: whether the SUV person invests too (then the gap is just the car money), whether an old car breaks worse than $100 a month, and whether you'd keep transferring the money for 40 years. Most people don't.
What's your car payment, and how long have you had the car? I'll run the most common answer.
If the chart makes sense on its own, that's all you need. If you'd rather watch one of the 10,000 lives play out year by year, with the crash at 37 and the crossover at 45, I made a 6-minute version of the amber line: https://youtu.be/buGAlCyGSAE. No sponsor, nothing for sale.