Gotta love Debian now

[hakerdefo]

I would repeat what I said in my previous post, it's most likely a dbus related thingy. You can easily test this by not using display manager, (LightDM) in this case, and logging in by good old 'startx' and repeat the same static IP steps and see if X goes down. My guess it won't.
Systemd not to be blamed here, IMO.
Cheers!!!

[jeffreyC]

^ It sure seems that way to me...

But it is awfully easy to blame systemd for debian bullshit.. they are after all partners

What has me wondering is that one of the many, many things systemd has taken over is network discovery and when you changed the network settings is when the trouble started.
Having PID 1 aka systemd responsible for the whole OS is what makes me think that when one part fails or does not work as it is programmed to expect then the whole OS is more likely to fail.