Have you ever considered that after 20 years of fixed interest, the follow-up loan is re-evaluated and if at 68 years old there are still €280k outstanding, there might be no loan or only with a nasty risk surcharge? It's not as simple as you imagine. It's the same as if the income situation changes negatively at the time of refinancing and there is only one income left.
However, the evaluation should also take into account that you then have a condominium that (hopefully still?) is worth a million and is three-quarters paid off. The money you have repaid over the previous 20 years will presumably not be considered "gone" in the evaluation. So if you still have €280k outstanding but also have €600k in property value, the risk for the bank should be manageable.
Unless the big real estate bubble bursts, and the condominium is no longer worth a million. Then the OP has a problem. He, and many many others too. But that is a completely different issue.