It all comes down to if GW can make money out of the army. That's the truth... and that's it.
Don't tell me that GW can't support another army... if there's money to be made... GW will hire more staff for the army.
I'm tire of people saying that GW can't keep up... blah blah blah.... the truth is GW can do anything if they can make profit out of it. I'm guessing that GW is maxed out in their limitation because of investment vs. return, so they only release one army/codex at a time. If GW did the math and they can make more money by releasing 3 codexes and 3 army books all at the same time..... trust me they would. They don't because (I would hope) this is their best business practice and it worked for GW.
Another thing is GW wouldn't release any new armies if that new army (Araby) will take away a chunk of their current army buyers. Example... (I'm being lazy here).... if they make (per Quarter) 1 million dollars with Empire and their cost of R&D, production, etc... cost them 300k, they make 700k from Empire. If they make Araby and it cost GW 100k and they make only 200k, but they notice that Empire only make 800k this time... thus... GW knows that a lot of players just switch from Empire to Araby... so now that quarter GW only make 600k w/ Araby and Empire vs. just Empire 700k.
New army must also bring in new customers base not old customer base who buys GW stuff anyway.
I think this is why GW only release one army/codex at a time... because they will always make money about the same each quarter and their customers base are about the same each month.
I think Araby would work if Araby will bring in new customers.... maybe when GW is thinking about expanding their company to the middle east or new market place....that would be a good idea.