As a programmer, who has programmed both games, and mods for games, although, not specifically for x-plane, I think your basis for the argument that it would cost you as much development time for the support of additional OSs, as it did to develop the base product is incorrect. You should not be redoing the bulk of your development, in both art assets and programming assets for each platform. If you are redoing everything completely from scratch, you are clearly doing it very, very wrong.