[Ekaros]

Other limitation that I know is that for tap water minimum is 55C. So the last building in loop should get at least that much. So 40C is only acceptable back at power plant.

In the end it comes to balance with cost, simplicity, capacity and such. Heat pumps do allow extracting heat from colder storage medium. But on other hand electric heating elements and heat exchangers are very much simpler and cheaper.

[fulafel]

In the good systems there's two pipes, return gets its own (like eg the earlier linked PDF describes), so it's fine for buildings to output cooler water to the return pipe.

But yes heat pumps are used in some parts as well in DH.