Etihad starts in 2003. Pilots who joined EK in 2003 are about seniority 600ish.
If they therefore just judged seniority on start date, the top guy in EY would be about 600 in EK- Still pretty good, arguably better that being in EY, and still top dog in "United Emirates" (or whatever) Abu Dhabi base. If they did something like a "3 for 1" merge, everyone would be about where they were in terms of position and upgrade potential. Either way, it would be a lot less bloody than the attempts to merge lists in the US and Canada, and in any case, seniority doesn't effect much here anyway.
I've also often said it only made sense for EK an EY to merge at some point, the UAE simply doesn't need two airlines.
And if we then have an airline with two bases, why not more......