I don't think it's very useful to think about which group of users would see themselves unjustly treated. This is about an economic calculation. Making the Mac version free now will create the perception that the windows version should be free as well if it's ever created, preventing it from seeing the light of day if it turns out to be viable at a price. Now if the windows version isn't viable even at a price, then I don't think it should stop the Mac version (either free or paid) from existing. In that case windows users should simply understand that the development costs for their platform are much too high to justify it. If neither version is a sound business proposition, then neither ought to be created, if both are, great, but if the mac version makes sense from a business perspective while the windows one doesn't at any possible price/demand level, then I don't see why the latter should hinder the former.