This is so bad for Frostmourne its not even funny. When the merge with Icecrown and Blackrock happened on season 1 of FM, when ICC started, I saw an instant decrease in FM people doing BG's. It feels unfair, imbalanced, and like your work means nothing when people can get what you have, but easier, and you're forced to play against them. I was part of a heroic raiding guild in FM1, and spoke with people from many other guilds, and they all stopped queing BG's after the merge.
You are tone deaf to how much of players' enjoyment is gotten from gearing up and smashing lower geared players, that are working their way up. If I put in the effort farming 25m hardmodes, push arena rating for gear, and max out professions and enchants, I want to be able to 1v3 in BG's since the other players don't go to that extent.
This whole argument was based upon Season 1 merging, which was BG-only. This is even worse if it means arenas are cross-realm. By trying to save Blackrock, you are sacrificing FM PVP. I was thinking of joining FM again for early seasons PVP, but now that thought is gone for real. Such a shame and a waste.