I think anyone with a pulse will agree that if you don’t have more than 1 planet after 10 days…you’re inactive.
The problem is that we base it off of account logins, not empire logins. So it is possible for a user to log into one galaxy, ignore a second galaxy, and be safe from removal from both.
This is understandably not ideal, but also not a high priority to fix compared to other issues.
If you feel differently, feel free to vote for it here: Inactive Checks by Empire
Reminds everyone to one planet wonder in sn
30 mill pop on one planet! 
Yeah but they might of joined and are playing in other galaxies
But I love the messages I get when I do nothing lol, I will try to stay there the whole round, that would be fun. 
Earlier in Supernova, my other fam member was a 1-planet inactive. After a few attempts to reach out, I then warned him I would mark him inactive. Then I did so, then I took him out. Pretty easy to take care of.