[BurningPenguin]

Maybe because 90%* of all houses need to be refitted.

(*estimated number out of thin air, but yes majority is gas or oil)

[rich_sasha]

In the UK, air source heat pumps are pushed as a 1:1 replacement, with running costs basically the same as gas, in GBP.

I can’t verify that easily, but this doesn’t seem disputed, so the retrofitting is maybe not so much of an issue. There is of course the initial installation cost, but also presumably lower maintenance costs.