The issue is that modern heat pumps are way more efficient than just a resistor. Not something that was obvious in Nakamoto’s day. Even gas furnaces are more efficient due to transmission losses.

If your house is one of the few that have an electric furnace, there’s no difference. For most people, though, there’s a huge difference. Running an appliance such as a computer to heat a house will cost more and be worse for the environment.

Wasn't it obvious in 2007 (or even 2000)?

A French company made a product along these lines already, but I don’t think it’s been updated since 2018: https://blog.qarnot.com/introducing-the-qc-1-crypto-heater/

It’s true this is as good as resistive heating in terms of heating efficiency, but it is still far less efficient than a heat pump. Unless you are in an area with a large excess of renewable energy it probably doesn’t make sense environmentally to use an asic heater.

One might use a smaller version as a space heater, rather than for primary heating. In that context it isn't really competing with heat pumps; space heaters are usually resistive.

It looks like the same company still makes some products that leverage waste heat from computation (https://qalway.com/fr), just not ones that specifically mention crypto as the source of the computation.