S!
I think open architecture is good for some of the games that have it. In Fs2000 for example, the sheer volume of aircraft out there makes it impossible for MS to include them all. Even going back to WW2 sims, there are just a HUGE amount of plane types and variations on types that a sim would never get finished if they were all to be included. But..
For WWI there are only so many planes. Yes there will be those few that want to fly a plane type or prototype that had 2 at an aerodrome for 2 weeks =), but most of us wont. Rather I know that 17 hours is triving to include ALL the relevant aircraft from the war. And done right. So the question then becomes, what would a third party need to add? The reason we have so many third party mods for RB is because they left a lot out. 17 hours will be covering these things and therefore shouldnt require open architecture to add/create missing items.