In my opinion, all guilds have this sort of problem - almost all player will, sooner or later, get tired of DDO, and 'finish' it - let's face it, there's only so many times you can level your guys up doing the same adventures, right?
That's why small guilds shoot themselves in the foot, in my opinion - unless they are die-hard uber-fans, sooner or later, they will leave and find another game.
It's just something you have to deal with. We have to deal with it, too; as a large guild, it happens a lot - people lose interest, and move on. We have a buffer, you see - and a larger guild, in my opinion, will help people stay longer, as they decide to play with their friends. Hey, I could be wrong - but on the other hand, I'm also not worried about new games coming out and destroying our guild; our base of players is enough I am pretty sure it won't happen!
New games come out all the time. If you're going to insist on a having a tiny guild, you're going to need to make sure they are hardcore ddo addicts who will never, ever leave.
Or, you might as well join a big guild
When GW2 came out, yeah, a few people went to play that for a while... and then came back
Since we were a big guild, it was no problem for us, or for them - they came back to welcoming arms, ready to play with their friends again....