God we are back to the X SM army could just be rolled into Codex: SM idiocy again are we?
Could they do it? Yes. Will they do it? No. Should they do it? Probably not. Why? Because its really smart from a bussiness POV and the good investement to turn over ratio of any SM release actually benefits the non-SM armies.
Just be thankful that GW have with the 5th ed SM codexes added new units to make them more unique....and guess what they will do it with DA and BT as well. I'm sorry but anyone who seriously thinks that the hobby would be better if they folded DA and or BT into codex SM instead of keeping it the way it is where they represent the potential for additional SM focused releases is a *****.