In the Danish system, each mortgage is backed by a matching bond. Thus, mortgage holders have two ways to pay off their mortgage: 1) pay the monthly payments or 2) buy the matching bond and, in effect, extinguish the mortgage. The latter option is valuable because when interest rates rise, the price of mortgages fall.
Instead of the complexity of issuing these bonds, would it not be a lot simpler to just allow borrowers to buy back their mortgage at its current value, which is what any other buyer of that loan would do. Mortgage loans are bought and sold all the time, for their current value. Just allow the original borrower be able to do that.
It is interesting that banks dont already offer this for a fee. Im not too knowledgeable on the topic, but wonder if it has do with how mortgages are bundled, and the cost/paperwork of unwinding that.