But there's an additional problem to the server population misguidings.
While the population is "medium", and that is likely not a false assumption, what it also does not take into account is the server's faction balancing. As an example, I will use my old server, Skullcrusher (US).
Skullcrusher is a medium population server. There is decent activity around peak hours. However, the server's faction imbalance leans favorably towards the Horde. If you look at the guild progression in terms of raiding on the server, you will see what I mean.
Of 59 guilds that have recorded any progress in current tier content, nine of them are Alliance guilds. Of those guilds, I can confirm 100% that one of them (Wrath) has ceased all raiding activity for months. So, the actual list is eight out of fifty-eight guilds that are on the Alliance side. All of these guilds currently are 10-man guilds. Horde has one 25-man guild confirmed (Reckless).
Therefore, based simply on raid size alone, the population of raiders is 80 to 515. This is not factoring in other circumstances such as additional raid teams any of these guilds may have and bench players.
Now, if you roll on Skullcrusher, which is listed as a medium population server, you are actually getting two different server types in one. If you roll Horde, the population actually is "medium". However, if you roll Alliance, the population description would be "low", and that's probably an overestimation.
Not only does Blizzard need to bite the bullet on merges (I don't care if you don't want to change your name, other people want to play the game the way it should be played...you can deal, honestly, and your name is probably stupid anyway), but they need to do it in a way to fix faction imbalances. If you merged Skullcrusher with, say, Blackwing Lair (a low population server with a very heavy Horde imbalance), it would do nothing for either server as a whole, it would simply bring more Horde together. However, if you merged it with say, Dark Iron (which has a heavier Alliance population), it would give greater overall benefit to both sides and fix all of the glaring issues at once.