OS X may be built on open source, but that is not the point.
1.) The original point was that Windows retains dominance because software developers concentrate on that market. Just because Apple supplies it's OS to developers doesn't mean that they choose to develop for that OS. When you say that "most will run seamlessly", is that a given, or do I need technical knowledge to get it working? If so, once again you've lost Joe Average.
2.) You have not addressed my 2nd point at all ... personally I think this is a MUCH bigger obstacle to OS X achieving significant market share.
Of course they choose to develop for that OS. To be a developer for OS X you have to sign up first and be accepted as a developer then you are either given a pre release copy of OS X to work on at Apple's World Wide Developers Conference or download it from their developer website.
Runs seamlessly is pretty self explanatory - running applications.
To cover your second point, how did you first get to use Windows? You had to or eventually had to buy it or it came with your new PC. It was your choice to go with Windows. If you decide to try out OS X there are ways to do this. Go to their website where it perfectly explains the transition from PC to Mac or visit an Apple shop all over SA. But THIS IS YOUR choice if it is something you want to do. You only remain Joe Average if you don't try new things.
I don't deny that Windows retains dominance in the market and I believe it always will however statistically OS X increases market share year after year. FACT.