I already wrote 4-5% annuity. Earlier, in the early 2000s/2010s, it was even a "solid" 5%. That was considered healthy back then.
Nowadays, no one thinks about that because the amounts are so high and no one dares to commit to such a high repayment anymore, and salaries have not risen as sharply in the last 5 years as the loans. Besides, people still want to consume and live as before.
Unfortunately, 5% is unrealistic for many people nowadays. With a rate of €1800, that corresponds to €432,000. Here in the south, you can't build much or anything with that.
With a low risk tolerance, 4% is certainly appropriate, I agree with you there. But with their €700k, the two won't manage without both continuing to work 100%.
One suggestion has already been made: downsize.
But there are alternatives (with the resulting consequences). Hence the suggestion to reduce the loan amount through more equity, etc., then they could go for 3.5% and roughly reach their €1800 rate.
I would want to avoid the full €700k in this scenario.