The mechanics
A new car loses 15–25% of value in year one; a standard loan amortizes slowly at first. Small down payments plus long terms mean the loan balance exceeds the value — sometimes for years. Roll that into the next purchase and you are financing interest on money already lost, compounding the mistake across vehicles.
The cost at trade-in
Negative equity at trade-in becomes either cash due or a bigger loan. A $5,000 underwater roll-up on a 60-month loan at 7% adds ~$99/month and ~$940 of interest to the next car — before that car's own depreciation starts. The affordability calculator's down-payment input exists to prevent exactly this.
Climbing out and staying out
Extra principal until positive equity (the early-payoff calculator shows the timeline), bigger down payments on the next purchase, shorter terms, and — hardest but most effective — keeping cars longer. The 84-month loan is not a financing product; it is a subscription to being underwater.