I used to believe that Blizzard made a mistake by starting raiding with a 10-man and then forcing guilds to move to a 25-man model. I thought that running two Kara groups and having to increase in size would be too much work and organization for most guilds, and would prevent the majority of guilds from really experiencing raid content.
I think I was wrong.
The data that has forced me to change my mind comes from WowJutsu. WowJutsu is an automated ranking site that crawls through the Armory. It notes when new loot from raids appears on characters and uses that to determine which guilds have killed which bosses. It's not perfect, but it is fairly accurate for the most part.
On the side, they have an automated list showing what percentage of raiding guilds have killed each boss. For example, only 3.37% of raid guilds have killed a boss in Black Temple. By comparing the number of guilds on two bosses that come right after each other in progression, we can see which bosses guilds are "stuck" on.
When examining the 10 to 25-man transition, we look at the end bosses of Karazhan and the first boss of Gruul's Lair, High King Maulgar. First, we make an assumption:
Assumption 1: If your guild has killed Maulgar, your guild has killed Nightbane.
This is a pretty reasonable assumption to make, in my opinion. From WowJutsu, we see that 74.51% of all raid guilds have beaten Nightbane, and 72.68% of all raid guilds have beaten High King Maulgar. This means that the vast majority of guilds who beat Nightbane go on to beat High King Maulgar. That they are able to overcome organizational challenge of getting 25 raiders. The conversion rate is a whopping 97.5%.
But maybe the assumption is wrong. Maybe there's a number of guilds who have killed HKM, but not Nightbane, and that's skewing our results. So let's soften it to:
Assumption 2: If your guild has killed Maulgar, your guild has killed Prince Malchezzar.
Honestly, if your guild has killed HKM, there is zero excuse for Prince Malchezzar to still be alive. Now we see that 88.88% of guilds have killed Prince Malchezzar. Again, the majority of guilds who beat Malchezzar go on to beat HKM. The conversion rate here is 81.8%. That's lower, but it's still very reasonable. 80% of all raiding guilds are able to navigate the transition from 10-man to 25-man.
I think that this is a reasonable result. It's not perfect, but between the Nightbane and Prince Malchezzar numbers, it's evidence that going from a 10-man to 25-mans was not a mistake, and the transition is within the capabilities of most guilds.