George,
Well, if Colt doesn't sell any SAAs and Ruger sells a ton of Blackhawks, does it? It would acknowledge that the basic design has appeal, but not that anyone likes it enough to pay the big price for the real deal.
Consumers buy on price point. When they look at Walther PPs and see the price tag they end up looking at other $500 options - and usually buy one of them. When they have less than $200 to spend, the PP knockoffs are good competition for the other cheap guns. In neither case is the real Walther the more attractive option, which is why sales of the real Walther are down.
Your argument is kind of like saying that Pargo (fake) flooring proves that hard wood floors are more popular than ever.
Cheap carry pieces popular (whatever the design).
Small 9 and .40 pieces are popular.
The Walthers are neither.