I share your enthusiasm about heat pumps, but I wonder what the efficiency of using waste heat is. Couldn't it be competitive with heat pumps? As it's a waste product, isn't it reasonable to also expect it to be more than 100% efficient?

You can’t extract energy from heat by itself. Only from a heat delta.

Think of heat like flowing water or charge. Only an altitude or voltage delta creates the flow needed to harvest energy.

You get no useful energy from heat you are already trying to shed because you have no delta to work with. (The entire problem exists because there is no surrounding environment with high heat capacity and lower heat.)

As a rule of thumb (obviously it varies) you spend about 1% pumping water round a heat network. So your CoP is around 99 if you consider heat truly free. It's actually higher as pump energy largely is converted to friction/heat.

Much more than 100% since the only energy you need to put in is for pumping the hot water around.