The MX5 is still the number one worldwide:
MX5 production 2006 ~ 48,000 units (PRHT included)
Kappa production 2006 ~ 35,000 units (Sol, Sky, Opel)
Both manufactures sell/will sell worldwide, so IMO it's not correct to have a look only in one area of the world.
With the Kappa's there was the first US manufactured roadster at the price level of the Mazda; therefore Mazda lost several roadster customers changing to GM. Same in Europe, there are a lot of people which never buy foreign cars.
As Bob Lutz said: the roadster market is limited.....
Mazda could see this developement and they tried to force their sales in other parts of the world.
One reaction could be the new hardtop version which ought to increase sales especially in Europe where retractables are very common. Time will show...
BTW we discussed something similar some days ago in this thread
MX5 long time production and sales charts are included.
Anyway the MX5 is very dependable because the long years of production experience, but the Sol and the Sky are the better looking cars