Large tech companies care about their reputation. If they dropped their own fork and went back and started again from Elastic that would be admitting their own incompetence. So not happening.
I think the bigger risk to Amazon: what if Elastic wants to pull the rug out from under them once again? Why take that risk when Amazon is already stable? Given the duration since the fork, I suspect there are more than a couple of features differences between the products that would have to be smoothed over.
