Let's say they'd have to shell out $400k to pay off the mortgage. Their $2000/month payment would be gone, but if we assume an average of 6-8% ROI on the $400k,

Let's say they'd have to shell out $400k to pay off the mortgage. Their $2000/month payment would be gone, but if we assume an average of 6-8% ROI on the $400k, they are missing out on a return of $2000-$2600/month. 20 years from now, the $400k becomes $1.282.000. Minus taxes.