The unions are probably one reason the Doug Parker didn't go after NW in his merger madness. Anyone who tries to link up with NW will have to deal with this. Since US is already having union issues in bringing its two parts together, trying to integrate a third unionized force was probably seen as nearly insermountable.
I will be curious to see hos a DL/NW merger deals with this. De-unionizing is certainly an option. My suspicition is that they might begin with the AF/KL model and work out the integration issues over a period of years, where NW employees would have a chance to see the advantages of the non-union DL force (or vice versa) to ulimately determine whether the integrated force would be union or non-union.