I personally believe we'll still be hearing about FSX for many years to come, just like we still can see a lot of FS9 users around. As long as we have active payware producers on the market (like PMDG itself, which is about to release their B777), a faithfull base of users and freeware developers, and online servers like IVAO and VATSIM up and running, FSX is safe from becoming history.
Maybe Laminar Research and Lockheed Martin will release better versions for X-Plane and Prepar3D, or even an unexpected developer can come around with something to beat FSX. Even if that day comes, FSX users will probably stick to their tons of add-ons, and most likely keep flying in more than just one simulator.
Long story short, it depends more on the users than on Microsoft itself.
