In the netherlands we were used to really cheap gas (groningen gas field). This meant that, instead of adding more insulation, it was cheaper to just install a bigger gas powered heater to heat our homes.

This means that people now really need to improve insulation of their homes big time, before even being able to consider switching to a heat pump.

My house has a 30kW gas heater. When I switched to a heat pump, first I had to replace the entire roof with PIR insulation. It was previously just uninsulated wood. Now I can easily heat my home with a 7kW heat pump (which most of the time doesn't even need to produce 7kW of heat).

I was lucky, my house already had insulated walls and floors, and used floor heating (low temperature heating). Most homes here use high temperature radiators, which become way less efficient when used with low temperatures produced by heat pumps.

So for many people here, it is cost prohibitive to switch to a heat pump, as they first need to improve isolation and replace their radiators for a heat pump to even work.

Not directly with the heat source, but with the output flow temperature that the heat source can create.

In that regard, an air-to-water heat pump is much lower temperature than a gas or oil boiler can efficiently produce.

That can cause a need for larger radiators to compensate and sometimes for insulation if the radiators can’t be further increased in size. (Or if the heat loss at design temp is too high for available or sensible sized equipment.)