just FYI, aptitude can identify the dependency chain between two packages. I don't know if it always finds the shortest chain, the most direct chain (choosing Depends over Recommends over Suggests), or just any chain, but on this machine (Ubuntu Xenial, the only one I can access right now):

  $ aptitude why network-manager systemd
  p   network-manager Depends libpam-systemd
  i   libpam-systemd  Depends systemd (= 229-4ubuntu21.29)

  $ aptitude why dbus systemd
  i   dbus            Depends    adduser
  i   adduser         Depends    debconf | debconf-2.0
  i   debconf         Recommends apt-utils (>= 0.5.1)
  i   apt-utils       Depends    apt (= 1.2.32ubuntu0.1)
  i   apt             Depends    gnupg | gnupg2
  i   gnupg2          Depends    gnupg-agent (= 2.1.11-6ubuntu2.1)
  i   gnupg-agent     Depends    pinentry-curses | pinentry
  p   pinentry-gnome3 Provides   pinentry
  p   pinentry-gnome3 Depends    libgtk-3-0 (>= 3.0.0)
  p   libgtk-3-0      Depends    libcolord2 (>= 0.1.10)
  p   libcolord2      Recommends colord
  p   colord          Depends    policykit-1 (>= 0.103)
  i   policykit-1     Depends    libpam-systemd
  i   libpam-systemd  Depends    systemd (= 229-4ubuntu21.29)

So yes, network-manager does seem to have a hard dependency on systemd, but dbus doesn't. Though I'm curious why network-manager should hard-depend on libpam...