First off, what about all existing Mac software? Seems like Steve did everything he could to make it easy to port OSXp to OSXi. If only the existing software were ported, the customer base is at least all the people currently using Macs plus the people who now might buy now because of the dual-boot security blanket, some of which might prefer the Mac environment more.
The reason why Macs have survived for so long in spite of their smaller market share (although their user share is significantly larger than 3%, more like 10-15%) is because the OSX user experience is compelling enough that people will pay extra for it. These people will not be happy with a software company that recommends booting into Windows to use their product. If there's a competing product on OSXi, the first company will suffer.
Conclusion: porting costs are low for existing titles, no worse than before for new titles. Titles that run natively will always do better in the Mac marketplace than ones that ask you to dual-boot, and that Mac marketplace will be bigger because of the dual-boot backup option.