Almost all loan agreements can be revoked - European Court of Justice

Only what they received in interest "too much" (because the interest rate, for example, fell in the meantime) would have to be returned. So they did not actually suffer heavy losses, but get what they would have received at the different points in time.

Well, if that were the case here (in my area there is no revocation, it's commercial), we would lose real money. The bank has to refinance your loan. And if they did that at 3% and get 4% from you, that works. If you pay back your loan, the bank still has the fixed refinancing, but today can only lend the money at 1%. Really shitty...

(since they are insured against something like that)

They are not!

So why shouldn’t I use it completely legally, just like I use a right of return for products I ordered but that don’t fit.

And then get upset if the bank terminates the loan due to a deterioration in economic conditions (e.g. the wife loses her job) ==> which, however, the bank kindly does not do, as long as the installments are still being paid.

And then get annoyed when the bank terminates the loan due to a deterioration in economic circumstances (e.g. wife loses her job) ==> which the bank, however, kindly does not do as long as the installments are still being paid.

Is a bank allowed to terminate the loan if you have not informed them about your economic changes? It is clear that the bank does not do this, I am just asking out of pure curiosity.

May a bank terminate the contract if one has not reported their financial changes?

The terms and conditions usually state that a change in creditworthiness must be reported and that the bank has all options in case of deterioration, ranging from increasing collateral to termination. Therefore, it cannot terminate due to "non-disclosure" (or is that a breach of contract for extraordinary termination? Lawyers would have to know), but due to the deterioration.

But in a serious case, that would be very vague, because the longer payments have been made, the fewer securities the bank would need.

The terms and conditions normally state that a change in creditworthiness must be reported and that the bank has all options from collateral reinforcement to termination in the event of a deterioration.

It's quite strange that such a clause is missing in the terms and conditions for fixed deposits placed at the bank. That is, that the fixed deposit may be unilaterally terminated as soon as circumstances regarding the bank's creditworthiness deteriorate. And that the bank is obliged to inform the customer of this. A classic example of how an asymmetrical power relationship between bank and customer has been created through clever lobbying.

So that one is allowed to unilaterally terminate the fixed-term deposit as soon as circumstances deteriorate regarding the bank's creditworthiness. And that the bank is obliged to inform the customer about this.

Well, that's what deposit insurance is for. There is nothing like that for building financing, or have you taken out credit default insurance?
A classic example of how consumer protection organizations have created an asymmetrical power relationship between bank and customer through skillful lobbying.