That is unfortunately a question of sport design and individual psychology that I can not even fathom answering. That being said, here are the issues we face with transfers: Skewed Alliance/Horde ratios resulting in boom-bust servers. (Stalagg, Skeram, and now Incendius).Real money trading using server moves. Moving low level characters for $25 from a deceased host where funds are simple to farm to a higher pop server that is more competitive. People use these transfers to mule around major ticket items such as edgemasters and cloudkeeper legplates in a bid to wow classic gold grow a golden vendor's stock of gold on servers which are more difficult to farm gold that the conventional way.
AQ War Effort. Botting and Multiboxing (That is already a huge post, and I believe that one deserves attention, just not here). The next point deserves some explanation, and that I hope the Devs are taking proper actions to remove this possible issue. When the war effort starts there will be. This is nothing but a math issue that is already solved weeks in advance. And with spreadsheets and types it makes it super simple to track all the necessary materials.
For example, Firemaw (EU) has estimated turn ins per day and goals they will need to hit until July 15th. It's looking very promising for them to be able to do it on day 1. Why are they doing this you ask? Since they are gamers playing a game which was our for decades now. It's similar to asking: why are people still speed jogging Super Mario? Because they could. They and a few other servers are going to have the war effort done and the Gates of Ahn'Qiraj opened one week when it comes out.
The reason for concern is that you're taking a look at an expected boom-bust situation again if we use history as some principle. The only response to this is really a warfare effort that is time-gated. I'm all ears, if there are other solutions.
Quite frankly, when discharging transfers that are free much care needs to be taken. It has torpedoed two servers that I have played now, and I'm tired of losing sleep because I need to play 4D chess with host choice on a normal basis. I've lost countless friends who are fed up with having to pay more income for problems created by Blizzard. That which I find as a remedy is faction based host locks. You cannot move to a server using a personality that's part of this faction with more 60 percent of their entire player population. Total servers are secured indefinitely. And there's friends. If they want to move to cheap wow gold classic a locked server then make a very simple requirement of being Actual ID friends for more than a month.