When you get a heat-pump you need the backup for really cold (-20c). So you just leave the existing one in for that case.
In my area of the US for example, gas costs 32￠/therm, which equates to 1.1￠/kWh.
Electricity on the other hand costs 6.4￠-10.9￠/kWh (depending on time of day), and while a third of it comes from renewables, another third of it comes from coal, so it ends up being net worse for the environment than just burning the gas.
Hum, I do not know in you country but here in France the maximum you can get from the grid is 36kW, 12kW per phase, 3 phases. And while the electricity is still far cheaper than in Germany if you try to heat a classic badly insulated home (who have a typical 34-40kW gas heating) with electricity you final bill will be a bit astronomical....
So 48-96kw, technically.
Installing a pump without installing insulation (if you need it) is silly.
Classic homes here are built with rocks and mortar, laid down on grownd, often stitched together in villages to spare a wall or two. As a result they are essentially impossible to insulate: insulating from the outside means agree with others for a job often as expensive al living the home for a new one. From the inside it's even more costly and not always doable since old homes tend to be irregular and with low ceiling resulting a too much internal space loss, not counting the effect of having loss much thermal mass. Toll buildings from just after WWII to the sixties are in even worse shapes, at that time almost no one care about insulation or thermal design because heating was veeeeeery cheap. The old rule in winter was "if it's too hot just open a window, it's cheaper than a thermostat".
These buildings are the MAJORITY of the buildings and their own tend to be not so rich so they'll reach both natural and economical EOL soon and non one have a viable idea of what to do. Rebuilding anything is possible in mere theoretical terms, but in 30+ years, not tomorrow, and only with a public hand keeping costs low and plan with a bit of design instead of building a new real estate rush with similar results of those of the recent past in few decades. In practice though it's next to impossible because vast majority do not want to relocate. In south EU most home owners are middle class or even some low incomes, not just few rich and all other rent from big society.
That's the real point. For EVs and p.v. it's essentially the same: those who live in modern homes have both in mean or can have at least. Almost all others can't. Those who lives in cities have already big parking issues since cities was build far before cars, so with not much room for them, and as well no room for reasonable domestic p.v. systems. Classic homes tend to be badly oriented and many still have no garages. As a result most can only recharge cars from public stations who are spread enough not too few but most are 22kW 3-phase as homes, so you need to find one, occupy it for maybe 4+ hours, move the car thereafter to live the place for another and find a parking for yours now charged. Something stressful enough that most who do not have a private garage simply do not buy EVs.
Evolving civil infrastructures is not such a quick and easy things, especially in dense areas and during economic/social crisis of various kind.
Also, opening a window if it's too hot (in winter) was never a rule here, we must save a penny wherever we can ;)