I found one of the reports that pits the nail in the development coffin read below:
http://steamcommunity.com/app/314160/discussions/1/611696927917689040/#p2I quote:
Microsoft remains the developer of FSX and is solely responsible for implementation of any features or content. Meanwhile, Dovetail Games is acting as a publisher for Microsoft and is only licenced to modify the simulator in order to make it Steam compatible. Although we have optimised FSX: Steam Edition for use on Steam, in accordance with our licencing agreement with Microsoft we are not at liberty to make significant functionality changes to the simulator.
We know that the SDK is highly important to the FSX developer community and that the current situation regarding its use with FSX: Steam Edition has not been made clear. Implementation of the SDK currently falls outside of our agreement with Microsoft and therefore we have brought your enquiry to their attention so that we can best provide you with the information that you require. This has meant that we have been unable to address your enquiry as quickly as we would like. We are sorry for any frustration this may have caused.
Microsoft has now issued us with clarification on this matter. FSX: Steam Edition does not include a developer SDK and the focus for this release is to provide players with a stable standalone simulator which utilises the advantages of the Steam platform. If you wish to use the SDK it remains available in the boxed edition of FSX and all functionality will continue to work as it did prior to the release of FSX: Steam Edition. There are no plans to introduce a SDK to the Steam edition at this time."
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This is quite a serious development (no pun intended) and it seems we have been mis-sold the product. Basically, those wishing to partake in the developer community (or even tweak a few items for our own requirements) will be unable to do so unless they already own the original boxed version.
My father said "Rember nine out of ten people around you are stupid, and I am not sure about you either" RIP . . .