They put it forward as a possibility but I doubt the merge will actually come to pass.
Consider that even right now, the populations of Deathwing and Ragnaros added together at peak time is around 7-8k. Then consider also that at that time, there are 2000 people in the queue waiting for their turn to play on Lordaeron. Now try and think about how many other players there are who'd love nothing more than to come play on Lordaeron, but are dissuaded by the lack of RDF, the low leveling rates, and the insane queue times. Now finally, factor in all the old players who'd come back to Ragnaros/Deathwing to check out the new core in all its glory, and also some of the current Lordaeron players who are fed up with 1x rate and want to get to the end-game ASAP.
If they merged Deathwing and Ragnaros, the cluster**** that would ensue when Lordaeron's core is ported over would be BEYOND anything the likes of which Lordaeron players are experiencing now. Not only would queue times be ridiculous for non-donors, but the lag from a server supporting 10k people that Lordaeron players often experience would hit everyone on the new Ragwing really hard. On top of that, Warmane would be seriously limiting its potential growth. A server capped at 10k with awful wait times would turn away a LOT of potential new players, whereas two servers with, optimistically speaking, maybe 6-7k players each could continue to keep taking in new players for a while before reaching their capacity. Keeping in mind here that Deathwing and Ragnaros are probably responsible for a large portion of Warmane's income, it follows as a logical consequence that keeping those two realms separate, such that they're able to host a larger amount of total players, is therefore the most financially prudent decision.