It will never be financially viable...
The Earth is flat...
Currently, here in Munich, the admission of the spherical shape is preventing one of the largest municipal buildings – because the city simply continues to say, "The Earth is flat, prove me otherwise." A municipal building is to be planned, the client is the city, the user is the city – but the two floors are to be used by different city institutions. All good, heating and ventilation planning is finished – but regarding the energy recovered from the exhaust air (which, according to Musketier, does not exist, but in reality is huge), it is currently not possible in the ventilation system to distinguish whether it originates from institution A or institution B. The city’s proposal: Then just leave out the ventilation system with heat recovery! Ha, very clever idea, now not only will the operation become massively more expensive but also the construction, because then heating etc. must be designed completely differently.
So yes, in the early Middle Ages, similar mental contortions were used; back then, it was considered normal to spend one’s life crawling, sitting, and coughing in the house. At some point, people saw that those with the newfangled fireplaces in the house could also stand, lie in bed, and generally have hobbies other than coughing. This insight is still pending for the window ventilators.