Except that you are kind of wrong about this. These API's are backward compatible as far as I'm aware. Like I mentioned before, something written for Direct X 10 will work on lesser versions, it just won't have some of the extra visual effects that DirectX 10 introduced.
No,
you are wrong.
However, Direct3D 10 is not backward compatible like prior versions of DirectX. The same game will not be compatible with both Direct3D 10 and Direct3D 9 or below. Games would need to be developed for both APIs, one version for Direct3D 9 and below if targeting Windows versions prior to Windows Vista and another version using Direct3D 10 if targeting only Windows Vista.
Again it isn't a big deal... my point was just to find something. Most developers simply write the program twice, once for 10, once for 9 using two different APIs. To the customer, it is "backward compatible"... but in reality it (Direct 3D 10) is not. Developers could do the same thing for OS X if they wanted to as well… there just is no point since people migrate faster on OSX than from XP to Vista.
Again, it’s a dumb point to make, it’s wrong, and it’s a dumb argument really.