The problem goes both ways, exactly.
Like I said, another problem is definitely the apathetic members and the ones who don't realize how important GS is to their guild, who then drop guild attempting to move onto one of the ones where they wouldn't have to work b/c 5-6 members really run the whole guild and make it impossible to fall below 300 GS unless they're on.
We can't make people mission, and definitely cannot make idiots avoid death by making another guild mad. Easiest thing to do is some form of cap on leaving/joining another guild. Especially one with too many GOOD members. TBH, since 85~110 players is average, maybe the better suggestion is actually changing the ways guilds and guild entry is handled entirely.
For example, instead, everyone could be born guildless, and no guilds have a train rate of 1x. 100 GS then qualifies as 2x, 200 as 3x, 300 as 4x. Guild Masters could instead be chosen based on players who would be reliable leaders (Example: Wiz), rather than Class (Random noob comes on during a weekend, gets free exp, auto-B classes, and takes a GS spot...has happened). Then the recruiting fun and guild expansion starts.
Finally, each Guild caps @ 25~30 members, except dark guilds (Roughly a quarter of the average PB).