I notice you done completely skipped over the change from 40-man raiding ta 10/25 what came in TBC. If'n yer arguements was correct, should there not have been a drop in subscriptions then? Afters all, it hit a lotta raidin' guilds hard, and hardest hit were the 15-or-so grunts what (accordin' ta conventional wisdom) was bein' carried. Leaders, not so much. So how's come the subscription numbers went up?
I also notice what you says 10-man raidin' came "into vogue" in mid/late-Rash. What does this mean? It started showin' up on magazine covers? I mean, 10-mans was more popular in terms of numbers of buggers startin' in Kara, and this never changed. 25-mans had better rewards all through the end of Rash of the Itch King, an' still offer a greater quantity. So is yer "in vogue" claim fer that point in time based on anythin' objective, or did ya fall inta a trap of "well, that's when subscriptions done peaked, so that musta been when 10-mans became more stylin'."?
I don't think that the drop from 40 to 25 really changed the "nature" of guilds. To put it another way, a 40-man guild is closer in structure to a 25-guild than a 25-man is to 10-man. A 25-man guild will maintain sub-groups like healers and tanks and ranged dps. But a 10-man generally only has the main group, with no subgroups. A 25 is far more likely to have formal loot structures, where a 10 will be mostly ad hoc.
Qualitatively, I would say that their is a point where a guild flips from "large" to "small" and that point is somewhere between 10 and 25. An interesting question might be 15-mans. Would they feel more like 10s or 25s?
Keep in mind that the guild needs more than the bare minimum. A 25-man guild is really more like 35 people, and a 40-man is closer to 50 or 60.
As for the timing, I don't think that Karazhan changed the nature of guilds. Everyone knew that future raids in TBC would be 25 man, so they temporarily bent their structure to accomodate Kara.
But I definitely think that around Ulduar, guilds started to internalize the idea that 10s were viable. Structure started shifting to accommodating 10s. Large guilds started forming subgroups, almost subguilds, that ran the 10s together.
In my view, it all comes down to structure. The structure of 40s is very similar to the structure of 25s. But that is different than the structure of 10s. Ulduar-ish was the time that guild structure started changing permanently, at least from my observations.
I just think the 40/25 structure was more conducive to retention than the 10 structure is.