It might be anecdotal but allow me to give a little information about the reasons for the population some.
Back in TBC, me and my friends were killing it, loved the city raids, loved battlegrounds, world PvP, and running heroics and instances.
Then when WotLK hit, by the end of it, none of us were left. I quit just before ICC came out waiting for WotLK to become fun before I finally gave up with 1 of my friend actually going till the end but he didn't bother with PvP anymore and was just there for daily heroics and raiding and logged off.
We had plenty who were excited to WotLK because of how good TBC was and were wondering how much better they could get it only to be let down.
PvP died in WotLK, ARENA numbers picked up but world PvP was dead, city raids was dead except for 1 run for most people just for a title, battlegrounds were beyond dead. As a feral druid I could kill a mage in 3 seconds if I got the intro on them unless they were geared. PvP in WotLK was balanced in the sense that everyone was a nuclear bomb and you can kill anyone if 5 seconds or less unless they were a paladin or death knight. Up until near the end of WotLK, a blues and greens paladin of various specs could kill raid geared players unless they were based around keeping range. And when I said I could kill a mage in 3 seconds that was no exaggeration as I could have my intro crit and take them from 100% down to around 3% alone and my crit rate was well over 55%.
WotLK was only good if you only did arena and raids and sucked out loud if you enjoyed anything outside of those.