That's also an option. But with people like Jana, that definitely was not the case. Everything that was available through the low rate was wasted.
The option to invest instead of repaying works out mathematically if, after taxes and possibly other costs (e.g., broker), you generate a higher return than the interest cost. Let's assume 1.5% for a 20-year fixed interest rate; then with a 25% capital gains tax, you would need a return of more than 2%. That is quite realistic—just the dividend for the DAX is roughly in that range.
But the devil is in the details. If a major stock market crash occurs at the wrong time, it can take several years to recover. If interest rates are also higher than originally planned, it can backfire. Personally, I would prefer not to commit to having €300,000 at a specific point in time if possible.
I am skeptical about advice from the bank, like with any advice where the advisor is not paid directly. In the end, banks have an interest in selling their own actively managed funds with front loads and a 2% management fee. By comparison, my DAX ETF has an expense ratio of 0.09% per year and the S&P500 ETF even 0.07%. It would be all half as bad if active management actually delivered better returns, but there are several studies on this and the results look pretty bad for fund managers.
How one handles equity and repayment is up to each individual. What is clear is that investing money at a higher rate than the financing interest only works if you take risks. Those who now choose very low repayment rates are mostly speculating that inflation will rise and largely devalue the loan in 20 years. My plan is to repay at about 3% or just under so that the loan is paid off within a reasonable time through the installments. That is somewhat lower than the originally planned 3.5%, so a kind of middle ground between repayment and investing. Prepayments are not really planned now. With those, of course, there is always the question of where the money should even come from in a substantial amount.
The loan-to-value ratio also plays a strong role in the equity used. The less you use, the higher the risk for the bank and the more expensive it gets. According to my contact at Interhyp, there are also banks that allow debt over equity beyond a certain loan-to-value ratio so that you can already start repaying during the construction phase. I find that relatively attractive as I think I can manage the double burden for a few months.