[jabl]

The trend in district heating systems is towards lower temperatures. Lower temperatures means that more of the waste heat in the power plant can be extracted, lower heat losses in the piping, and also if using heat pumps then lower temperatures can give a big efficiency boost. But yes, the downside is higher massflow, but this is certainly an optimization problem that engineers have calculated the optimal point for, given the available technology.

Steam heating is an ingenious invention, but it was also made in a world where pumps, the electricity to run them, PID controllers and whatnot, were expensive. That's why you tend to find it in the oldest district heating grids around.