But as to Steam - FSX, I've had a doubt or two
For the most part it's been "How do they plan on making money?"
Not that I'm complaining about their right to earn a living. But how will that affect me?
Well, while searching for something totally different I came upon this Steam forum post - RISE OF FLIGHT.
It's a little old, but may still be relevant.
Of particular interest was this question -

And the answer was -

So unless I'm mis-reading it - What you have now in FSX will probably not work in Steam: FSX.
Sorry, but there's no way I would want FSX without Orbx/FTX. Nor would I want to pay for it a second time.
Remember Microsoft Flight?
They would let you buy add-on scenery & aircraft that they developed.
And unfortunately almost none of the flight sim hardware would work.
Even if Steam does do some updating of FSX, I doubt that it'll hold any interest for me.
As the post on the Steam forum states: "The Steam version is aimed at new users ..."
I'm betting that it'll do as well as the video game, Microsoft Flight did.