How do you suggest to keep buildings livable during the decades that it takes to (as you suggest) essentially rebuild cities?

By switching from burning Russian hydrocarbons for heat to heat pumps so you can heat and cool while being more efficient? This isn't a rebuild, it is a moderate adaptation to the actual requirements to live in the Europe of today.

For steam based heat systems there is also cooling options, so that is a fix for those cities with central heat service.