This particular feature could be (quite easily!) implemented without much help from the daemons themselves, at the cost of duplicating the config option of list

This particular feature could be (quite easily!) implemented without much help from the daemons themselves, at the cost of duplicating the config option of listen address. To make the matter funnier, this is doable with mechanisms that were adopted in unices in '90s. This is why systemd underdelivers on its promise of automatic dependency detection.