Ubuntu 18.04 Will No Longer Do Automatic Suspend By Default Unless On Battery
Fortunately I wasn't the only one to think that out-of-the-box behavior was outright horrible. This ended up being a change that trickled in via upstream GNOME 3.28.
Yep, same awful default behavior when trying Debian Testing with GNOME 3.28... Discovered that when SSH'ed into a box while the system was originally sitting just as the GDM log-in screen, wondering if benchmarks were stressing the system too hard...
Fortunately, due to multiple issues stemming from this behavior, Ubuntu 18.04 is changing the behavior per this bug report. Now the automatic suspend will not be enabled by default unless the user is running on battery power.