I think Des & Jiffy have pretty much nailed it with that statement.
IMHO another contributing factor is the disappearance of the most popular such as the Albatrossen, Fokkers & Sopwiths. I know WNW always said their kits were limited run but the market has changed dramatically since they made that statement.
I'm sure if they changed policy and reintroduced the more popular sold out kits no one would complainand a steady flow of buyers would continue. I'd for one would buy and build Albatros D series kits till they came out my ears, I'm kind of hoping Merit would do a 1/24th one, even if it were sketchy I'd buy at least 2 to kit bash.
With a tighter budget not so good eyesight & motor skills and little coming in WWI I'm re-exploring other aspects of modelling too.
For me there's no real excitement about what and if WNW will release these days, I'm not knocking their policies but recently it's done nothing for me personally, gone are the days of checking their website frequently and I've found their lack of communication disheartening not excitingly secretive anymore. I'm sure though if WNW hit is with a surprise Albatros W.4 or a Junkers D.I or one of the Austro Hungarian fold or similar my interest will rise again but for now I'm slumbering (still building my Junkers J.I though slowly.

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I don't mean the above to sound as negative as it probably does.
Keith