gradually renovating is probably not an option for us if we want to change fundamental things like installing underfloor heating and controlled residential ventilation.
There are measures that are not feasible "while in operation." This certainly includes underfloor heating, controlled residential ventilation, but also electrical or water/sewage systems, or for example, larger floor plan adjustments and interior plastering. However, there are also things that can be easily renewed while the property is occupied. These include, for example, windows or doors, a new boiler/heat pump, the roof, the facade, and even a bathroom can be temporarily unusable if (as is usually the case) an alternative option is available.
Personally, I do not see why renovation should regularly be more expensive than new construction. It depends entirely on the purchase price of the specific property. If the seller wants to charge a premium for the well-maintained but no longer desired 1970s interior, of course, you have a problem. Otherwise, I would offset the costs of a shell construction of the same size against the property price, add demolition costs, and subtract the outdoor facilities. This should normally at least result in a break-even outcome.
If you also consider that the location is generally nicer, some old buildings have their own charm, and basically, you no longer have to bear the entire risk of new construction until the shell acceptance (delays, botched work, insolvency, etc.), there is usually little argument against an old building.