I think when BFA concludes the factions will still exist (so a orc can't just roll into SW or vice versa), but the player characters will be above the petty squable. We are heroes of Azeroth after all not just our respective factions.
It will allow players regardless of faction to group together (unless in Warmode)
It will allow (in theory) for more zones/quests to be developed since they won't have to develope seperate things for the factions.
Will reduce the number of mounts created that would turn into a net gain over all (hopefuly).
Might help WPVP, since both factions will be in the same area
This will allow Blizzard to progress the story cause then faction balance won't be imperitive. That is the big problem w/the faction war being a story telling point. The story can never really go anywhere cause of faction ballance.