This is a "compilation" of a few web sources (see at bottom) of how to completely remove systemd from a Debian 8 system while retaining desktop functionality (with the likely exception of Gnome).

I did this to my LXDE Jessie system, and so far all works well.

** Only try this at your own risk! **

• First, install sysv-init:

  apt-get install sysvinit-core sysvinit sysvinit-utils

• Reboot

• Purge systemd:

  apt-get remove --purge --auto-remove systemd

• Prevent installation of systemd packages:

  /bin/echo -e 'Package: *systemd*\nPin: origin ""\nPin-Priority: -1' > /etc/apt/preferences.d/systemd

• Add the "angband repo" to your repo lists:

  /bin/echo -e 'deb http://angband.pl/debian nosystemd main\ndeb-src http://angband.pl/debian nosystemd main' > /etc/apt/sources.list.d/angband.list

• Save and add the repo's signing key from http://angband.pl/deb/archive.html

  apt-key add <the saved keyfile>

  apt-get update; apt-get dist-upgrade

• Search for any systemd "leftovers", and purge them:

  dpkg --get-selections | grep systemd

  apt-get remove --purge --auto-remove <remaining systemd packages>

• http://without-systemd.org/wiki/index.php/How_to_remove_systemd_from_a_Debian_jessie/sid_installation
• http://lkcl.net/reports/removing_systemd_from_debian/
• https://lists.debian.org/debian-devel/2015/02/msg00189.html