The Legion has already gotten pretty big: established members thus far include
the six shown hiding Cyclops’s parts, the real Sun Boy, plus Colossal Boy, Invisible Kid, Star Boy, and of course Superboy and Supergirl.
The idea of hiding a disassembled superweapon in the past to prevent it being misused is a bit weird at first glance, but also pretty clever. Anyone looking for this thing in the 30th century has to contend with a thousand years of time and natural forces having destroyed all traces of evidence showing where it was buried. On the flip side, someone in the 20th century might stumble across one of the pieces, but they wouldn’t have any clue what they’d discovered- especially since they’d be trying to understand the purpose of an artifact that’s actually only one-sixth of a complete device. The plan is really only vulnerable to attack by someone like the impostor Sun Boy, who has a list of the parts’ locations (… somehow) and the help of Superboy to retrieve them.
The weapon itself, unfortunately, is just an incredibly morally reductive concept. The idea doesn’t even have any interesting qualities or implications like that thing about Saturn Queen having been an upstanding citizen on Saturn did.