Hey, very simple idea here...
The WWII set if not "complete", is very, very, very full at least. There are holes yes, but not gaping ones. Most of the planes that could conceivably be added to it now would be snapshot/scenario specialists, not really popular rides in any of the MAs.
The WWI set, is by comparison, ALL holes. France, one of the major combatants, is not represented by a single plane. And it is guaranteed that such additions as the Spad or the Se5 would see heavy daily use on their own merit in the WWI MA. Furthermore, with a few more planes, we could actually stage WWI AvA/scenario setups.