Given the challenges involved in retrofits this would also be an enormouse sink for labor and hence employment. The vector of money would need to change, which of course is why modernization of this kind is opposed by legacy money holes such as firms selling products intended to be burned as rapidly as possible (an ideal situation for any vendor!).
"Individual building-level approaches are needed to understand the full potential of rooftop photovoltaics (PV) at national and regional scale. Here we use the European Digital Building Stock Model R2025, an open-access building-level database, to assess rooftop solar potential for each of the 271 million buildings in the European Union. The results show that potential capacity could reach 2.3 TWp (1,822 GWp residential, 519 GWp non-residential), with an annual output of 2,750 TWh based on current PV technology. This corresponds to approximately 40% of electricity demand in a 100% renewable scenario for 2050. Already by 2030, over a half of buildings with floor area larger than 2,000 m2 could generate most of remaining capacity for the 2030 target with 355 GWp. Across member states, non-residential rooftops could cover 50% or more of their PV targets, with several exceeding 95%."

Mapping Europe’s rooftop photovoltaic potential with a building-level database - Nature Energy
This resource presents an open per-building dataset of rooftop solar photovoltaics potential for the European Union. The results show that potential capacity could cover approximately 40% of electricity demand in a 100% renewable scenario for 2050.
