That's not quite correct though: all of my Debian systems (jessie and stretch) are happily running with either sysvinit or openrc. Maybe there's a problem running Gnome without systemd, but I'm not using that anyway.

The only thing the TC decided is that daemons must be able to run under systemd -- which can be achieved by providing a (cross-init) sysv script, or a systemd service file.

I am not sure what is your definition of "won". There is only one option can be supported, and debian picked systemd over other options. It seems exactly fits the definition of "won".