Well, systemd has certainly not been accused of choosing nothing too often. Rather the opposite.
If only there were some other DHCPv6 client that we could use.
I meant that perhaps noone else came forward to do the IPv6 DHCP module coding for systemd-networkd (or whichever sub-module)? That's a question.
Rather than asking whether isc-dhcp-server over SysV, or whatever was better.
Without adjudication, it was a choice between this and init (the daemon it replaced) at a minimum.
systemd-networkd isn't a replacement for init, it's a replacement for NetworkManager/netctl/….
It's not even really a replacement for NetworkManager; it's designed to be used to provide basic networking within containers, for systemd-nspawn. It can be used on host systems, but it's deliberately fairly lightweight and designed for fairly simple configurations in containers, rather than as a full fledged replacement for other networking management daemons.