They never needed the xel'naga to return. The xel'naga were just a plot device to explain the origins and relation of the zerg and protoss. The backstory could have been written without them. In fact, if the backstory had been written without them then I think the story would have gone in a completely different direction that may or may not have been better than what we got. Or, more cynically, Blizzard would have retconned them into the story if the zerg and protoss had previously been established as independent.
I should probably get back to writing my reboot narrative fanfiction rather than constantly criticizing Blizzard. Feel free to critique what little I published so far, maybe make suggestions as to where the plot should go. I have an outline sketched out for the major events on the small scale and the sector scale, but the nitty gritty isn't set in stone and I've already veered off my outline and had to edit it more than a few times.