Is an assignment clause mandatory?

If genuine payment problems occur after 50% loan repayment, then

A) the bank is still the lesser problem or
B) any other bank is happy about a mortgage loan financing

If real payment problems occur after 50% loan repayment, then

A) the bank is still the lesser problem or
B) any other bank would be happy to provide a mortgage loan

Real payment problems would arise, for example, in our case if I were to be without work long-term (main earner) or otherwise long-term unavailable (serious illness). Then it could happen that we can no longer afford the long-term high installment.

However, you might be right. The way that another bank refinances the remaining 50% and thus the installment can be lowered could of course be the solution. However – it could become difficult to find a bank that finances the 50% (e.g., remaining 150,000 euros) if the main earner is long-term unemployed or, for example, occupationally disabled (regardless of whether the property serves as collateral with 100% potential market value).

It is not about interest rate risks at all.

A contract is and remains a contract. Binding for both parties and also for the "buyer".

And this is exactly where, I think, the misunderstanding lies with the questioners. Already in the initial post it is asked whether the buyer can call in the loan immediately and without reason.

He cannot. That means whoever pays on time is on the safe side during the fixed interest period.
So only if you rely on goodwill might you possibly have worse odds...

That hits the nail on the head in terms of meaning.

Many builders want the financing "at their place of residence," meaning they want to address their caseworker directly in case of problems and resolve issues or other matters face-to-face. This is one of the reasons why many builders reject financing through Interhyp & Co. with unknown or distant banks, even if these might be cheaper.

Precisely this motivation can, of course, fail in the event of an assignment if the (originally) financing bank sells the claim to a third party. This can happen in portfolios (a bundle of existing financings), which can consist of good and/or bad financings.

The clause is probably completely OK in the vast majority of cases. Foreclosures do not come out of nowhere, and courts in Germany still protect against arbitrary actions. Things are never as bad as certain TV shows would have us believe.