"only heat pumps can be more efficient"

Exactly. And if you have a whole house/apartment electric heating system it is very likely that it is in fact heat pump based. So you are better off using that for heat than the lights.

If your whole house/apartment heating system is gas or oil based, that will probably be even cheaper than the heat pump electric version.

>if you have a whole house/apartment electric heating system it is very likely that it is in fact heat pump based

Hmm, where? I have never knowingly been in a house with a heat pump in the UK, and lots of cheaper flats have electric radiators and immersion heaters.

Finite fossil fuels... which might not even be "environmentally cheaper" in the long run...

Not quite – some energy was lost converting from its source form to electricity. If you heat your house by directly burning natural gas (the most common way in cool countries like the UK) then it can still have an efficiency advantage over converting electricity to heat.

The fossil fuel infrastructure (even disregarding extraction), is not zero cost either.

This is true but - especially if your house is not well insulated - getting heat in the right place matters too - most people don't need to heat their roofs or ceiling spaces. There's also a more minor point on time of day you want heat or not.

Well, hot air tends to rise anyway.

Anyway, my point is that the inefficiency lost to "waste heat" is not 100% waste.

The only thing that can be more efficient is one of the most common forms of heating devices.

During the summer, it's also extra inefficient to pump 100W into a device producing heat and then pump even more power somewhere else to cool down the room.

There's a lot more people that use heating than cooling (in Western countries at least).

Although heating with gas will produce different amounts of CO2 than heating with electricity.