I dont think Strikeforce would disappear. That would be an error on Zuffa part. It makes complete sense to own the 2nd tier MMA organization. All we are seeing right now, are the UFC quality fighters that fought in Strikeforce, being moved over. Thats what Strikeforce is going to become. The testing ground. To merge in into the UFC would be to dilute the product quality. There may be popular fighters that make money, but are not good enough. To keep them in the UFC, would make the quality drop. To lose them to another org, would lose them money.
The only way to keep the UFC as the golden product with ALL the best fighters, and very little C level ones, is to control the feeder orgs, not merge with them.