Ever since word came out yesterday at Blizzcon that Mythic raiding (currently the 10m & 25m heroic raiding) was going to 25M, healers have been pretty concerned. After all, most high end raiding guilds are forced to go with 8-9 healers on the roster (some with 10 or more even) because Heroic Thok required that many healers, despite the fact that the majority of the other bosses in Siege of Orgrimmar require 4-6 healers.
At Blizzcon, the question was asked about raid comp and how many healers Blizzard expected 20m Mythic raiding guilds to run. They believe most raids will run with 5 healers, and for some of the top progression kills where they need to get as much DPS as possible, that you will be running 3 or 4 healers (or more realistically, I except we will see some 2 healer kills).
What does this mean for excess healers currently sitting on a top end 25m guild? I expect top guilds who want to min-max will probably keep 7 or 8 on the roster still, specifically for comp issues, because guilds want to be able to have 2 spirit links or 2 barriers. And they need to keep the healing roster up to keep doing full clears in Siege, especially if they are doing sell runs. However, there will be a lot of healers looking for guilds, so I expect guilds will be keeping a close eye on which healers are performing the best in Warlords of Draenor.
I expect we will see more healers needing to be flexible and have competitive DPS in order to keep up with gearing, when raids will have so many healers sitting on the bench. And we could see some main switch as well.
Don’t forget that you often have people who simply decide to quit by the time the new expansion hits, especially when you are looking at people needing to level 10 levels to 100. Some will be slower than others and others will decide to main switch (and I expect some concerned healers will), so even guilds going into the new expansion with 9 or 10 full time raider healers on the current roster now, it could just end up being 6-8 by the time you are ready to pull the first raid boss in Warlords of Draenor.