This is the core issue, all the focus is going into a single realm and if the other realms will have to wait for any upgrades until Lordaeron is finished then its going to take another 5 months~ with the current of release rate of content on Lordaeron.
Naxx/Os/EoT are scheduled for release today so that will last until next month if not more.
Then its going to be Ulduar for another month or two.
Then its going to be ToC/Onyxia.
Then ICC will follow next and likely RS will be last to be released a month after the Icecrown Citadel hits live.
If this is the case then it means that Cataclysm, Pandaria and other WoTLK realms will continue to steadily decline in population as they have been for the recent months due to lack of attention other than rare critical fixes to resolve stability issues. It might seem that MoP is doing fine right now, but its temporary due to the promised 5.4.8 that was supposed to be released 'soon' as stated in October 4, 2015
Everyone is hyped up and returning to the realm, population is healthy.
A minor setback, some unforeseen issues arise that push the update by estimated 2-3 weeks. It's fine this stuff happens all the time with everyone, community patiently waits and population grows along with the anticipation for the new patch and endgame content.
Hype is real and population is really healthy, guilds are rebuilding in preparation of conquering the newly released content.
2 weeks later
Players are slowly getting upset due to the broken promises.
December 14th
Devlog for Lordaeron comes out but the rest of the realms remain clueless.
Reason: Devs need sleep, Lordaerons changelogs and devlog required more time than anticipated.
This is the timeline so far. Days turn into weeks, weeks turn into months and now from the post made by Arbiterone we hear that Frostwolf will get work done on it only after Lordaeron is fully released, which will be late spring early summer.
This is the view from outside, and players patience is running low.