I love the WWI arena, but like Citabria said, sometimes one can't find even another soul in the arena, let alone a good dozen or more. It is a great break from the ordinary and most people in that arena have a desire to fight on a somewhat level playing field. Infact, everything Citabria said is spot on. I do hope they do some work to it to make it a little more appealing, I just hope any changes don't alter how the players in there conduct themselves.
As for the Triplanes dominating... Not really. It's truly about the pilot and aim. Get into a 2v2 and it's even more apparent.