in what other direction should it go? That interest rates will rise sharply in three years? That is reading from the crystal ball. In such a favorable situation as you have now, very few are able to live practically for free and save properly. How many can already save 1800 euros per month? If interest rates rise so much that 1800 euros per month no longer pay off, then we have a completely different problem here in Germany.
Well, if you do the math, you will find that continuing to save 1800 EUR can certainly be an overly optimistic calculation. At first, 1800€ per month sounds like a lot:
Assuming the TE’s production costs increase by 3% annually; then the interest rate for the loan must not rise
by more than 0.175% per year. Otherwise, the same rate as calculated for them today cannot be maintained (purely from interest, and with the goal of still being finished by the year 2051).
The only advantage in the consideration would be that the loan-to-value ratio will improve (~15% over the next 3 years, but that might only offset a possibly even higher interest rate increase).
(by the way, 1% interest on the balance already included).