First, let's start with a caveat. The following theory is most likely not true. I doubt its validity, mostly because I would like it to be true, and that is always a dangerous sign. But I present it as a hypothetical for your delectation.
The modern MMO decline in subscriptions, as evidenced by World of Warcraft and The Old Republic, is being caused by the shift in the extended endgame to smaller groups and smaller guilds.
Let's look at at World of Warcraft. The decline in subscriptions starts in mid-to-late Wrath, when 10-man raiding starts coming into vogue. In Cataclysm, when 10-man raiding moved up to first-class citizen standing, the decline accelerated.
In The Old Republic, the primary endgame group size is 8-man. It is clear that the SWTOR has real trouble holding on to people at max level. Small guilds, built for a small group endgame, are just not sticky enough for significant player retention.
The one MMO which has shown continuous growth, even over the same time period, is Eve Online. And Eve bucks this trend in group size. Endgame in Eve trends toward larger and larger groups, with hundred-person fleets flying around.
If this idea is true, why would large raids, and the corresponding large guilds, be stickier than small raids?
First, and most importantly, there's a lot more room for "grunts" in a large guild. Grunts are average players who like playing the game, but don't really want to take on extra responsibility like the officers. If you take a 30-man raid size, and break it into 10-mans, I think you end up losing the bottom 10 players, just because there really isn't room for them in the subsequent guilds that form. I think a lot of people just want to play the game, and are perfectly willing to follow orders from someone more dedicated.
Second, larger guilds and sub-communities are more likely to have people playing during off-hours. They can feel less lonely, which contributes to people sticking around.
Third, turnover is more easily managed by larger groups. People leaving and joining is not as much of a big change to the group. A larger guild is also more likely to be able to absorb a smaller group of players. This is also true when a guild breaks up. It is easier for three or four 30-man guilds to each absorb a faction of players from the dead guild. In contrast, asking a 10-man guild to absorb 5 players strains the resources of that guild.
Fourth, the intra-guild bonds don't have to be as strong in larger guild, as they are in a smaller more tightly-knit group. They don't require the large effort to create, to maintain, and don't cause as much damage when they break. I think this is actually an advantage for a lot of players. They just want a casual, light relationship with their guild, not an intense one. I think this also makes it easier to apply to a new guild, and form bonds which are "strong enough".
Fifth, large group endgame requires a greater focus on technical performance by the developers. Does anyone believe that the SWTOR engine could handle 40-man raids? Whereas WoW could handle that 8 years ago. If 40-man raids had been a development requirement, SWTOR would have had to optimize earlier and harder. And that would have a lot of trickle-down performance improvements, especially for low-end machines and the performance of the levelling experience.
For these reasons, I think larger groups are just stickier and better for the extended game than smaller groups. Smaller groups are easier on the officers and the devoted, definitely. However, I think they cause a lot of the grunts to be unable to find a home at endgame, and thus they unsubscribe and fade away.