The piece you're missing, beyond just "gas is cheap", is that increasing the heating capacity of a heat pump costs lots of money when installing it, whereas inc

The piece you're missing, beyond just "gas is cheap", is that increasing the heating capacity of a heat pump costs lots of money when installing it, whereas increasing the capacity of a gas boiler isn't much more expensive during installation, but costs loads more when running it. So technically it would be possible to install a heat pump with the same capacity as the existing gas boiler, but it would costs a ton more and it would all be upfront.