Loan agreement: Exclude cost-incurring sale of claims?

You are mixing up two things that can coincide but fundamentally have little to do with each other.

First, a bank always earns the same amount on a fixed interest loan, regardless of how interest rates develop.

A sale of receivables almost always has one of two reasons: to reduce risk or to increase liquidity (or both).

The borrower is only indirectly involved with both because the bank manages its own risk or liquidity with them.

Therefore, the sale of receivables is not a problem for the borrower at all. If he always adheres to the contract, nothing happens. And if he does not adhere to it, enforcement occurs. And in that case, it does not matter which bank or company enforces it. So nothing changes in that regard.

For other receivables, factoring (sale of receivables) is more the rule than the exception, of course without collateral by land charge.

And regarding the term and the correlated risk, you are not wrong, but also not right. You have ignored Par. 489 [Baugesetzbuch].

But this value is definitely not fixed in the contract, right?

Yes, it is; it is stated in the Building Code. And contracts in Germany are primarily the Building Code; the loan agreement itself is basically just the deviations from the Building Code and additional details; we are not in the Anglo-Saxon legal area. In that case, something like that would indeed have to be in the contract.

then maybe she wants to get rid of you earlier than at a 2.5% arrears...

By the way, 2.5% arrears on a 200K loan would be an arrears of 5,000€.

And even if the bank thinks in 10 or 15 years it could earn significantly more:

She cannot (any longer) get rid of you; even if the loan were sold, the buyer would have to assume the same rights and obligations.

) moreover, the probability is higher that after 10 years I have already repaid a lot...

That totally depends on how much repayment you have agreed on. You could also do 10 years with 1% repayment.

Of course, 10 years are cheaper than 20 years; but the risk of adjustment is also higher.

It's all a matter of personal taste.