Only the credit interest is usually negligible today.
Well, for that you then borrow your own money at expensive interest.
And repayment substitute is like coffee substitute: it’s okay, but tastes like sh... :-o.
In financing for owner-occupation, repayment should be the priority. Secondly, securing the remaining debt. In the example, one could calculate with a home savings contract of over 60k after 15 years and a new 100k annuity. For the home savings contract, about 30k would need to be saved in 15 years (2k p.a. or €167 p.m.). I would classify that as a “self-runner.”
Furthermore, here (with the low land value) the lending value should not be neglected either. High safety discounts from the bank will be to be expected. A house on land that is worth nothing is also worth nothing (not much). Another reason to watch the remaining debt.
Also, if all goes well, a high net income is available. That should be flexibly integrated into short-term opportunities. That is better placed in repayment than in savings at zero interest.