The problem is that most avid players have already reached lvl 80, and are mostly found in faction cities & northerend.
All other areas are populated by people leveling alts or new players, so the chances of meeting someone else has drasticaly reduced to near zero.
Outlands is a teensy bit better, but the players are most probably deathnights.
I doubt they will start merging servers as the servers are generally near capacity with the above said users, you can see by the server stats how full a server is.
Merging say only the Azeroth & Outlands realms of a battle group together may be a good idea as you will then have several servers users all out in the wild seeing each other, but, then main faction cities will be so full of people that the lag will be insane.
The lag can be countered by placing the trainers & auction houses in Dalaran, but that will spell instant doom to most traffic in pre northerend areas bar levelers as one would have so few reasons to go pre northerend, you'le just bounce between dalaran & the surrounding areas and dungeons.
At the end of the day, Blizzard can pretty much doom WOW with the next few lvl caps as the time it takes to level + pretty much soloing till you get to populated areas will be so boring, newcomers will quit while there ahead.
With the new battlenet account system, there probably looking at keeping payers who will migrate from wow to some other blizard mmo ....
Just my thoughts ...