But with 400€ per month, you would have 48,000€ in 10 years and not 100,000€, and nothing has been renovated, repaired, or set aside yet, and the incidental purchase costs are not included either. With the rent, however, everything is paid off. I find the objection quite reasonable.
On the topic: If I were you, I would also wait and save and see where things go in the next few years. We also bought early (at 27 and 29), but you are still quite a bit younger. In addition, in the long term, you will have significantly less money available with children (partial or complete loss of income plus significantly higher costs due to the children themselves). In your area, there should be affordable existing properties, I see that as more likely, and then you can sleep better. I do not necessarily see a new build here. But take a look at what happens in a few years and reassess the situation then.