First, remember, HTC doesn't care WHERE you fly. Early war, mid war, late war, AvA, DA, TA or WWI arena. As long as your having a good time, and keeping your acct active they are happy.
Simply stated more options means more happy customers, as your drawing from a bigger base.
Adding WWI is just one more string for the bow, one more different thing that you can do.
If you were paying attention the last couple patch's there have been HUGE changes in foundation stuff. Changes that may not effect us for a bit, but that set the stage for everything that follows.
They will start testing those with the WWI planes first, as they have a nice small set, 4 birds, and they can dial in all the bells and whistles.
Once that is all set then the really big job is of upgrading all the WWII planes to the new damage model with 4x as many damage points. That one will take time. But its much easier to do most of your testing in a small arena, with limited #'s of people, and only 4 planes. Then take what you learn from there and start putting it to work on the WWII planes.
The idea was also raised to have a once an "anything goes" arena where there would be no perked planes, or field capture, and all the evil con missions from the past could be flown and enjoyed.
Who wins, the Shark with laser beam or the super fokker?

I don't know about you, but to me these are great and exciting times to be flying here in AH. Lots of new stuff constantly in the pipeline.
Attaboy HTC! Rock on!