55 C might be enough to heat buildings but it's right on the edge where domestic hot water needs to be to kill Legionella. So traditional DH systems need to run hotter.

There low-temp/cold DH systems out there that rely on heatpumps in the buildings to extract the heat. Less losses in the network and you can even use them for cooling, but needs heat pumps everywhere.

In comparison, a traditional heat exhanger is pretty simple technology; just a hunk of metal with a valve.