Chris Murphy composed on 2016-08-30 16:13 (UTC-0600):
Anything that overrides default.target is supposed to be temporary. To
Not everything gets used as it's "supposed to be" used. With a dozen or more installations spread across dozens of machines, a digit in a stanza constitutes a keyword in answering the question "what will happen if I proceed with this" when looking at the selected boot menu item.
make it the default, and therefore maintain compatibility with system upgrades:
systemctl set-default multi-user.target
That was probably done already. I'm still waiting to see evidence that the upgrade process is actually proceeding, at least 35 minutes after having "Reached target System Update" show up on the screen.
In effect the upgrade fails because there are two conflicting temporary default overrides: /system-updates and the 3.