The rates usually don't go up because the lender got greedy and felt like screwing you harder. If the mortgage rate doubles, that's because your money is now worth less than it used to be. This also likely means that your income is going to grow soon enough, so you wouldn't have much trouble repaying your loan. While your remaining loan is still nominally $N, if you took a loan today for the remaining part using the paid-off equity as collateral, you could likely get twice as much and the lender would proceed accordingly.