In cases 2) (which is as I see and from experience even impossible i.e. end up in 3) since removing upstart installs systemd-sysv automatically) and 3) kernel boot line init= switch is not required to use SystemD. At least that is my experience as I wrote repeatedly. *)
In case 3) You loose more than You've displayed**):
Code:
:~$ sudo apt-get purge systemd-shim -s
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
indicator-datetime* systemd-shim* ubuntu-desktop* unity-control-center*
unity-control-center-signon* webaccounts-extension-common*
xul-ext-webaccounts*
0 upgraded, 0 newly installed, 7 to remove and 10 not upgraded.
Purg xul-ext-webaccounts [0.5-0ubuntu2]
Purg webaccounts-extension-common [0.5-0ubuntu2]
Purg unity-control-center-signon [0.1.7~+14.10.20140814-0ubuntu1]
Purg ubuntu-desktop [1.327]
Purg unity-control-center [14.10.0+14.10.20140922-0ubuntu2]
Purg indicator-datetime [13.10.0+14.10.20141009-0ubuntu1]
Purg systemd-shim [8-4]
As I've wrote, there are several packages that are to be renewed in order to be able to do what You're proposing and since we are even before toolchain I see nothing more appropriate than a little patience.
Update₁: For sake of testing I've performed „downgrade“ on one machine installing again UpStart (which removed SystemDSysV and I've reinstalled UReadAhead, FriendlyRecovery which were removed when removing UpStart) and I kind of cornered myself while painting the floor since SystemDSysV is now uninstallable due to 209/215 version dependency conundrum explained earlier. That looks more like present situation in vanilla Vivid install, it works nicely and awaits solution in proposed repositora about the faith of SystemD. Just thought that You should know, not having to go through all that Yourself. Yes, systemD is not fully ready for Vivid but it is well and kicking as is UpStart also. In futire days I think we will see full removal of Upstart, at the time when SystemD penetrated all neccessary packages and when all interconnections are ready. Greeting from Upstart machine that I've painted myself into the corner on.
Update₂: Just to clarify, not to leave impression that anybody else trying aforementioned would really corner h{im,er}self. SystemDSysV could be DL-ed (version -1) and used to get again hold of UpStart free install etc. Or, simply, vivid-proposed could be defeated for a while, apt database updated, and -1 version of SystemDSysV would be applied in all further apt doings until vivid-proposed is enabled again or that package get into regular repos. Just got some (rare) spare time and thought to disperse possible FUD I might have unintentionally started.
*Note: I do have vivid-proposed enabled so all my aforementioned experiences are with that factor involved. Your mileage due to vivid-proposed disabled might vary.
**Note: This is true regardless of vivid-proposed enabled or not.
